226ra 230th excess generated in the lower crust implications for magma transport and storage time scales excesses of 226 ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs followed by fast ascent rates of magma from slab to surface up to 1000 m yr however we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle derived 226 ra excesses and so reduce inferred vertical transport rates we developed an incongruent continuous melting model and both the incongruent melting reaction and ingrowth effects contribute to the 226 ra excess in particular we found that dehydration melting of amphibolite can produce modeled 226 ra excesses greater than 300 mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 u excess subducted slab signature this amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements similar to those observed in several arc settings which may distinguish these magmas from those with 226 ra excesses produced by slab dewatering alone

recent contribution of sediments and fluids to the mantle s volatile budget subduction modifies the cycling of earth s volatile elements fluid rich sediments and hydrated oceanic lithosphere enter the convecting mantle at subduction zones some of the sediments and volatile components are released from the subducting slab promote mantle melting and are returned to the surface by volcanism the remainder continue into the deeper mantle quantification of the fate of these volatiles requires an understanding of both the nature and timing of fluid release and mantle melting 1 here we analyse the trace element and isotopic geochemistry of fragments of upper mantle rocks that were transported to the surface by volcanic eruptions above the batan island subduction zone philippines we find that the mantle fragments exhibit extreme disequilibrium between their u th ra isotopic ratios which we interpret to result from the interaction of wet sediment melts and slab derived fluids with rocks in the overlying mantle wedge we infer that wet sediments were delivered from the slab to the mantle wedge between 8 000 and 10 000 years ago whereas aqueous fluids were delivered separately much later we estimate that about 625 ppm of water is retained in the wedge a significant volume of water could therefore be delivered to the mantle transition zone at the base of the upper mantle or even to the deeper mantle as fluids and melts rise from subducting slabs they may interact with each other and the surrounding mantle wedge and may be contaminated or modified by shallow processes such as cumulate melting and crystal fractionation to separate the role of these possible contributing processes we need to constrain physical processes a significant number of short lived u series isotopes studies have been conducted on arc lavas and magma chamber residence times are quite well constrained 2 however conclusions about the timing of source metasomatism have varied 1100 kyr refs 26 and there has been debate about the

induced polarization response of zeolitic conglomerate and carbonaceous siltstone three case studies investigating induced polarization ip responses of a zeolite bearing conglomerate and of two carbonaceous siltstones are presented the ip response of these noneconomic geologic materials can either mask or mimic the response from sulfide mineralization which is sought by electrical field surveys the nonsulfide rock types which produced unusually high responses on ip field surveys were sampled by core drilling for chemical mineralogical and electrical laboratory study the electrical response of core samples was measured in a four electrode sample holder over the 0 03 1000 hz range geologic description of the core petrographic examination of thin sections mineral identification by x ray diffraction xrd and chemical analysis of samples supplemented the electrical measurements a surface phase response of 20 mrad was obtained from field surveys over the gila conglomerate at an arizona location core samples of the gila were examined in thin section and clast surfaces were foun

modification of heavy mineral assemblages in english coversands by acid pedochemical weathering summary weathering of non opaque heavy minerals in five english soil profiles developed in texturally homogeneous late devensian coversands has been investigated by spline interpolation of detailed mineral counts values for d90 90 mineral depletion derived from the splines suggest a depletion sequence that is generally applicable to moderately acid weathering of sediments apatite olivines amphiboles pyroxenes epidotes garnets more resistant minerals weathered minerals form three groups each of which is eliminated in an almost discrete stage weathering is accompanied by a characteristic developmental sequence of surface textures weathering rates of most of the silicate minerals are probably determined mainly by their fe2 contents effective surface areas and soil ph considerable variations in the depth and severity of weathering between profiles may reflect differences in vegetation histories heavy mineral weathering is related to soil chemistry to examine its implications for podzol genesis

partitioning of strontium between plagioclase and melt a comment abstract the origin of the partition coefficient d for sr in plagioclase is ascribed by blundy and wood 1991 mainly to the chemistry of the crystal rather than that of the liquid however a major data set used to support this conclusion also demonstrates an important correlation of d with the augite content of the liquid one truth cannot be used to deny another from the same data set moreover it appears that d has a limiting value of 1 0 in the system an ab confirming the importance of liquid composition the data base of natural magmas said to justify the conclusions of the paper omits the important troctolitic and noritic low augite varieties for which the augite effect was specifically claimed and therefore cannot be used to discount the effect the data base is skewed toward silicic igneous rocks for which a high value of d is experimentally known the attempt to rationalize the data from the kiglapait intrusion by supposing the escape of low sr material ignores the evidence of the chilled margin which matches the sr budget of the cumulates and neglects to mention that the vented mass would have to be equal to that of the entire intrusion to account for the small variation in sr observed much work remains to be done but it is clear that the liquid plays a much more important role in this partitioning effect than allowed by the authors

a compositional approach to the reconstruction of geochemical processes involved in the evolution of holocene marine flooded coastal karst basins mljet island croatia abstract geochemical patterns in the environment are always the result of certain processes therefore it is essential to decipher a process to properly evaluate the environmental role and potential of chemical elements compounds this allows the distinction between natural and anthropogenic influence on elemental concentrations however if the compositional nature of geochemical data is neglected erroneous or misleading conclusions regarding the processes involved are probable in this study the reconstruction of depositional environments and processes through the holocene in two sediment cores obtained from submerged sinkholes located on the island of mljet croatia was performed by taking into account the compositional nature of geochemical mineralogical and grainsize data problems involving compositional data are always multivariate for example the concentration of a single element does not carry any interpretative information as only the ratios between elements do this has led to the discovery of a large number of geochemical proxies based on elemental ratios which describe certain environmental conditions and processes involved nevertheless some proxies have been found to be restricted to only some specific environments thus preventing them from being used in general therefore some kind of relation between different proxies is necessary to obtain final conclusions however when using simple elemental ratios those correlations cannot be obtained due to the nature of compositional data with a sequential binary partition of a compositional vector orthonormal log ratio olr coordinates proxies can be constructed when based on expert knowledge those proxies fully acknowledge the geochemical properties of the chosen elements with one major difference that the correlation between newly obtained variables is mathematically well grounded as a result the final conclusion is more accurate in this research geochemical proxies obtained as a representation in olr coordinates of the elements that are enriched compared to the local soil were used to perform principal component analyses it helped to unravel the evolution of sedimentary environments mineralogical xrd and heavy mineral data and grain size analyses supported the conclusions obtained based solely on geochemical data furthermore data analysis suggests that the proxies for redox conditions described in the literature should be used with caution as their use is somewhat limited

reactive bulk assimilation a model for crust mantle mixing in silicic magmas bulk assimilation of small millimeters to 1 km fragments of crust driven and ultimately masked by reactions during xenolith melting and magma crystallization is an important mechanism for crust mantle mixing xenoliths containing mica or amphibole undergo dehydration melting when incorporated into a host magma yielding mainly plagioclase pyroxene fe ti oxides and hydrous melt the xenolith is physically compromised by partial melting and begins to disintegrate xenolithic melt and crystals are mixed into the host magma xenocrystic zircon is liberated at this stage the cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions the reverse of dehydration melting all pyroxenes and oxides phenocrysts xenocrysts or crystals having a hybrid signature will be subject to these reactions producing feldspars amphiboles and micas that incorporate material from several sources a particularly effective mixing mechanism implicit in the model is a reduced energy penalty for bulk assimilation much of the assimilant remains in solid form compared to melt assimilation models a large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas while the assimilation of low density crust and concomitant fractionation provide the isostatic impetus for ascent the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent

the loess section at borden kent se england a 5 3m profile originally described as loess with a buried interglacial soil is reinterpreted from petrographic evidence and thermoluminescence dating as a holocene colluvial accretionary soil mineralogical analyses of coarse silt 16 63 m fractions suggest that most of the silty colluvium was derived from weathered thanet beds upslope though some loess was incorporated during the final depositional phase thin sections show that clay illuviation occurred penecontem poraneously with deposition of colluvium thermoluminescence properties suggest partial optical bleaching of the grains during rapid deposition which is consistent with a holocene colluvial origin

comment on steady state 226ra 230th disequilibrium in mantle minerals implications for melt transport rates in island arcs by m d feineman and d j depaolo earth planet sci lett 215 2003 339 355

experimental determination of trace element partitioning between pargasite and a synthetic hydrous andesitic melt in order to more fully establish a basis for quantifying the role of amphibole in trace element fractionation processes we have measured pargasite silicate melt partitioning of a variety of trace elements rb ba nb ta hf zr ce nd sm yb including the first published values for u th and pb experiments conducted at 1000 c and 1 5 gpa yielded large crystals free of compositional zoning partition coefficients were found to be constant at total concentrations ranging from 1 to 100 ppm indicating henry s law is oparative over this interval comparison of partition coefficients measured in this study with previous determinations yields good agreement for similar compositions at comparable pressure and temperature the compatibility of u th and pb in amphibole decreases in the order pb th u partial melting or fractional crystallization of amphibole bearing assemblages will therefore result in the generation of excesses in238u activity relative to230th similar in magnitude to that produced by clinopyroxene the compatibility of pb in amphibole relative to u or th indicates that melt generation in the presence of residual amphibole will result in the long term enrichment in pb relative to u or th in the residue this process is therefore incapable of producing the depletion in pb relative to u or th inferred from the pb isotopic composition of morb and oib comparison of partition coefficients measured in this study with previous values for clinopyroxene allows some distinction to be made between expected trace element fractionations produced during dry cpx present and wet cpx amphibole present melting rb ba nb and ta are dramatically less compatible in clinopyroxene than in amphibole whereas th u hf and zr have similar compatibilities in both phases interelement fractionations such as dnb dba are also different for clinopyroxene and amphibole changes in certain ratios such as ba nb ba th and nb th within comagmatic suites may therefore offer a means to discern the loss of amphibole from the melting assemblage elastic strain theory is applied to the partitioning data after the approaches of beattie and blundy and wood and is used to predict amphibole melt partition coefficients at conditions of p t and composition other than those employed in this study given values of dca dti and dk from previous partitioning studies this approach yields amphibole melt trace element partition coefficients that reproduce measured values from the literature to within 40 45 this degree of reproducibility is considered reasonable given that model parameters are derived from partitioning relations involving iron and potassium free amphibole

origin of 226ra 230th disequilibria in arc lavas from southern chile and implications for magma transfer time abstract improved understanding of mantle melting processes and melt transport requires knowledge of how fast magma is generated and transferred from source region to surface the rate of magma transfer can in favorable cases be estimated from radioactive disequilibria between nuclides of the 238u series young lavas from southern chile in which 238u 230th disequilibria have been measured sigmarsson et al nature 346 1990 163 165 sigmarsson et al nature 394 1998 566 569 were analyzed for 226ra abundances the disequilibrium between 226ra and 230th in these lavas is found to correlate with 238u 230th disequilibria and 10be be morris et al nature 344 1990 31 36 these correlations strongly suggest that the excess of 226ra over 230th is due to the addition of a slab derived fluid to the magma source since ra and u are fluid mobile elements and the cosmogenic 10be is most likely derived from the subducting nazca plate beneath the andes the largest slab signature is observed in the lavas of villarrica volcano which is the most active volcano in south america a model for subduction fluxing is discussed in which the u series disequilibria in arc lavas will reflect the integrated dehydration process during metamorphism of the subducting plate and the metasomatized mantle but be principally controlled by the latest hydrous mineral breakdown in the mantle wedge repeated precipitation and dehydration mineral reactions of the hydrated mantle could be the homogenization process of the slab input needed to explain the 10be be b be correlation for different arcs morris et al nature 344 1990 31 36 the fact that excesses of 226ra and 238u over 230th are correlated indicates that linear arrays on the 230th 232th 238u 232th diagram are not isochrons reflecting time elapsed since a fluid addition but rather mixing lines between a fluid phase and melts the 226ra 230th disequilibrium in arc lavas suggests significantly shorter timescales for magma transfer or less than 8000 years this disequilibrium is consistent with minimum magma transfer rate through the mantle wedge on the order of 10 m year finally the correlations of 226ra 230th with 238u 232th and 10be be in andean magmas imply that magma chamber residence time is of the same order of magnitude beneath the stratovolcanoes studied

magmatic evolution of late cenozoic volcanic rocks of the lau ridge fiji three main groups of lavas are exposed on islands of the lau ridge the lau volcanic group lvg 14 0 5 4 ma are predominantly andesite korobasaga volcanic group kvg 4 4 2 4 ma are predominantly basalt and mago volcanic group mvg 2 0 0 3 ma are basalt hawaiite lvg and kvg lavas are mostly medium k tholeiitic rocks with high lile hfse ratios characteristic of islands ares while mvg lavas are ne normative alkalic rocks with high lile and hfse characteristic of ocean island basalts lvg lavas have high nd 8 0 8 4 and low 87sr 86sr 0 70273 0 70349 similar to n morb whereas kvg lavas have slightly more radiogenic values nd 7 5 8 4 87sr 86sr 0 70323 0 70397 mvg lavas form an isotopically distinct group having lower nd 4 6 4 9 and 87sr 86sr ranging from 0 70347 0 70375 lvg lavas were erupted in a primary oceanic island arc vitiaz arc during the miocene basaltic lavas were derived by approximately 19 partial melting of mantle wedge peridotite with only minor subduction component andesites and dacites were produced by low pressure plagioclase pyroxene titanomagnetite dominated crystal fractionation kvg lavas were erupted during the period immediately prior to or during the initial stages of rifting in the lau basin and like lvg lavas show significant chemical differences at the northern and southern ends of the lau ridge lavas at the northern end type ii appear to be derived from a more depleted source than lvg but with a greater amount of subduction component those at the southern end type i probably came from a slightly more enriched source with less subduction component mvg basalts and hawaiites were derived from an enriched mantle with little or no subduction input the hawaiites type i could not have been derived from the basalts type ii and the two magma types must have come from different sources indicating mantle heterogeneity the lack of subduction influence indicates the mvg lavas are tectonically unrelated to the present day tonga arc and the lack of depletion indicators suggests they have tapped a different new part of the mantle wedge this may reflect introduction of sub pacific mantle through the present tonga lau subduction system

u th pa ra study of the kamchatka arc new constraints on the genesis of arc lavas abstract the 238u 230th 226ra and 235u 231pa disequilibria have been measured by mass spectrometry in historic lavas from the kamchatka arc the samples come from three closely located volcanoes in the central kamchatka depression ckd the most active region of subducted related volcanism in the world the large excesses of 226ra over 230th found in the ckd lavas are believed to be linked to slab dehydration moreover the samples show the uncommon feature of 230th 238u activity ratios both lower and higher than 1 the u series disequilibria are characterized by binary trends between activity ratios with 231pa 235u ratios all 1 it is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting we suggest that they are more likely to reflect mixing between two end members a high magnesia basalt hmb end member with a clear slab fluid signature and a high alumina andesite haa end member reflecting the contribution of a slab derived melt the u th ra characteristics of the hmb end member can be explained either by a two step fluid addition with a time lag of 150 ka between each event or by continuous dehydration the inferred composition for the dehydrating slab is a phengite bearing eclogite equilibrium transport or dynamic melting can both account for 231pa excess over 235u in hmb end member nevertheless dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid derived 226ra 230th a continuous flux melting model is also tested in this model 231pa 235u is quickly dominated by fluid addition and for realistic extents of melting this process cannot account for 231pa 235u ratios as high as 1 6 as observed in the hmb end member the involvement of a melt derived from the subducted oceanic crust is more likely for explaining the haa end member compositions than crustal assimilation melting of the oceanic crust is believed to occur in presence of residual phengite and rutile resulting in no 226ra 230th disequilibrium and low 231pa excess over 235u in the high alumina andesites consequently it appears that high alumina andesites and high magnesia basalts have distinct origins the former being derived from melting of the subducted oceanic crust and the latter from hydrated mantle it seems that there is no genetic link between these two magma types in contrast with what was previously believed

the role of trace minerals in chemical weathering in a high elevation granitic watershed estibere france chemical and mineralogical evidence we investigated chemical weathering in a high elevation granitic environment in three selected watersheds located in the pyrenees france the sites were located on glacial deposits derived from similar hercynian 300 ma granites characterized by the occurrence of zoned plagioclases and trace calcic phases epidote prehnite sphene apatite the surface waters at those sites show high ca na molar ratios 1 which could not be explained by the dissolution of the major plagioclase oligoclase present in the rocks the coupled approach of investigating stream water chemistry and the mineralogy and chemistry of rocks and soils allowed us to explore the role of the weathering of trace calcic minerals in calcium export at the watershed scale the weathering of the trace calcic minerals which represent 1 of the total rock volume are responsible for more than 90 of the calcium export at the sites annual cationic fluxes 23 104 eq km2 yr calculated for the estibere watershed are among the highest reported for high elevation systems draining granitic rocks and 80 of this annual cationic flux can be attributed to the weathering of trace calcic phases calculations based on isotopic values 87sr 86sr go in the same direction except apatite the trace calcic phases appear to be mainly silicates thus the type of chemical weathering observed in the estibere watershed may have an influence on atmospheric co2 consumption by granite weathering however comparison with other watersheds draining granitic environments worldwide and with the two other sites in the pyrenees indicate that the role of trace calcic phases is important in most young environments exposed to chemical weathering e g high elevation catchments on glacial deposits other factors such as the date of glacial retreat the physical denudation rate the hydrological functioning of the watershed and the nature and structure of the soil cover are also important

pedogenic and geomorphic evolution of a buried dune palaeocatena at magilligan foreland northern ireland abstract field description and laboratory analyses of a complex stratigraphy recently exposed near magilligan point have elucidated the late holocene evolution of magilligan foreland n ireland a marine beach ridge palaeocatena passes northwards into a penecontemporaneous aeolian dune palaeocatena both include soils and peat filled depressions and are overlain by an aeolian dune sand a second dune sand is confined to the south of magilligan foreland the thickness of the decalcified zone of the palaeocatena varies from 0 4 m near the crest of the palaeodune to 2 m below its slopes the palaeodune core exhibits large scale cross stratification enclosing unusual small scale reverse flow cross stratified sets d 90 90 depletion values indicate similar weathering intensities in the palaeocatena and in the overlying sand but deeper weathering in the former a mineral stability sequence has been constructed and unweathered sand identified which indicates the original composition of the magilligan deposits inter profile differences in weathering rates are attributed to variations in local environments

contrasting timescales of crystallization and magma storage beneath the aleutian island arc abstract geologic chronologic and u th isotope data from pleistocene recent basaltic to rhyolitic lavas and their phenocrysts constrain the long term evolution of magmatic processes at seguam island aleutian island arc and suggest a model in which erupted magmas were derived from a single deep seated reservoir for 130 kyrs of its eruptive history the monotonic evolution in 230 th 232 th 0 ratios is consistent with radiogenic ingrowth of 230 th in a long lived magma reservoir between 142 and 9 ka internal u th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40 ar 39 ar dating which implies a short period i e 10 3 yrs or less of crystal residence in the magma prior to eruption these results can be reconciled if small batches of magma are repeatedly extracted from a deep thermally buffered basaltic reservoir followed by decompression driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering 12 km 2 the ignimbrite as well as subsequent rhyolitic and basaltic lava flows signal a decrease in 230 th 232 th 0 and 87 sr 86 sr ratios the abrupt change in magma composition at 9 ka most likely reflects disruption of the long lived reservoir by rapid ascent of basaltic magma with low 230 th 232 th and 87 sr 86 sr ratios and excess 226 ra while the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored the shift in isotopic composition and evidence for a protracted 100 kyr period of magma storage would have been missed had the focus been on only the most recent historical eruptions our results suggest that not all arc basalt produced in the mantle wedge separates and ascends rapidly magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano and u th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs provided a sufficiently long period of volcanic activity is examined

u series isotopes and destructive plate margin magma genesis in the lesser antilles high precision tims230th232th isotope ratios are presented from a suite of lavas which span the length of the lesser antilles island arc compositions vary from low k tholeiitic lavas with low87sr86sr in the north to high k calc alkaline to alkalic lavas with higher87sr86sr in the south the northern lavas are characterised by 230th238u 1 a three component model is required for magma genesis in the lesser antilles sediments from the subducting slab melt and variably enrich the mantle wedge while the subducting oceanic crust dehydrates releasing fluids enriched in u ba sr and k but with negligible th and low87sr86sr which promote partial melting in the wedge ta zr increases with87sr86sr southwards along the arc and this is inferred to reflect increasing enrichment of the mantle wedge through the addition of partial melts of subducted sediment mass balance calculations are consistent with a relatively constant fluid flux except for u and little variation in the degree of partial melting 10 15 along the arc and they suggest that the sediment contribution to the arc lavas increases from 2 in the north to 15 in the south in the northern parts of the arc the addition of u by these fluids overprints any increases inth u that might have been caused by partial melting and results in lavas with 230th238u 1 m yr 1 which imply channelled flow through the mantle wedge

the effects of magma replenishment processes on 238u 230th disequilibrium short lived isotope data are increasingly used to investigate magma chamber residence times arc lavas have excellent potential as tracers of magma evolution because their magmas are often out of u series isotope equilibrium and some arc magmas have yielded surprisingly old pre eruption u series ages from mineral separates a model is presented to examine the behaviour of u th isotopes for a range of conditions between closed system and steady state magmatic systems magmas can be maintained out of secular equilibrium for long periods of time by periodic mixing events with new magma batches the latter results in oscillations in 230th 232th consistent with the relatively constant values observed in lavas of different ages from some centres the effects of periodic reinjection of magma and mixing of old and young crystals on u series mineral isochrons are also considered mixtures of crystals produced by either replenished or closed system processes can readily result in linear arrays in many cases it may be valid to interpret these as minimum average residence times for the crystals whereas the liquid residence times calculated from u series data for periodically replenished systems are maximum values

the effect of assimilation fractional crystallization and ageing on u series disequilibria in subduction zone lavas abstract although most arc lavas have experienced significant magma differentiation the effect of the differentiation process on u series disequilibria is still poorly understood here we present a numerical model for simulating the effect of time dependent magma differentiation processes on u series disequilibria in lavas from convergent margins our model shows that in a closed system with fractional crystallization the ageing effect can decrease u series disequilibria via radioactive decay while in an open system both ageing and bulk assimilation of old crustal material serve to reduce the primary u series disequilibria in contrast with recharge of refresh magma significant 226ra excess in erupted lavas can be maintained even if the average residence time is longer than 8000 years the positive correlations of 226ra 230th between sr th or ba th in young lavas from convergent margins have been widely used as evidence of fluid addition generating the observed 226ra excess in subduction zones we assess to what extent the positive correlations of 226ra 230th with sr th and ba th observed in the tonga arc could reflect afc process results of our model show that these positive correlations can be produced during time dependent magma differentiation at shallow crustal levels specifically fractional crystallization of plagioclase and amphibole coupled with contemporaneous decay of 226ra can produce positive correlations between 226ra 230th and sr th or ba th to a lesser extent therefore the correlations of 226ra 230th with sr th and ba th cannot be used to unambiguously support the fluid addition model and the strength of previous conclusions regarding recent fluid addition and ultra fast ascent rates of arc magmas is significantly lessened

u th and ra disequilibria sr nd and pb isotope and trace element variations in sunda arc lavas predominance of a subducted sediment component the time scales of magma formation transfer and storage beneath island arc volcanoes are important because of the insights they provide into the physical processes of magma formation and evolution with possible links to the prediction of eruptive hazards here we present new mass spectrometric u th and ra and sr nd and pb isotope data along with major and trace element data from 19 lavas along the sunda arc indonesia the lavas range in sio2 from 49 to 75 and the combination of high sio2 and low 143nd 144nd indicates an important role for shallow level contamination by ancient crustal materials in sumatra diagrams of 143nd 144nd versus th ce and u th versus 87sr 86sr reveal negative trends which allow contributions from both subducted sediments and fluids to be identified the fluid component as expressed in elevated u th 238u 230th and 226ra 230th o ratios is ubiquitous in all of the lavas including near primary lavas from galunggung sr isotope data indicate that the fluids had variable composition and derived from altered oceanic crust and the u th and ra isotope data suggest that some fluid components must have been added less than 8 000 years ago tholeiitic lavas from flores have the lowest th contents in the arc and accordingly the highest 226ra 230th o and 238u 230th for these there is evidence that both u and ra addition probably occurred less than a few thousand years ago this suggests that the onset of melting was linked to fluid fluxing and cannot have been due to decompression alone in contrast the sediment component may have been added as a partial melt and sediment addition can account for the observed along arc variations in sr nd pb and th isotopes there is no requirement in our data for an oib component in the mantle wedge increases in sio2 in the galunggung lavas are accompanied by decreases in both 238u 230th and 226ra 230th o which are inferred to reflect mixing with melts of pre existing mafic lavas mixing and differentiation must have occurred faster than could be recorded by either the 238u 230th or 226ra 230th systems there is little evidence in our data for any effect of the australia indonesia collision on the composition of the lavas across arc trends in lava composition indicate an increasing relative contribution from subducted sediment a correspondingly weaker fluid signal and probable decreases in the degree of partial melting residual aluminous clinopyroxene or garnet may become important in controlling ree and u th fractionation in the far back arc

reappraisal of uranium series isotope data in kamchatka lavas implications for continental arc magma genesis uranium series isotopes can be used to determine constraints on the timescale of slab dehydration and melt production at subduction zones however interpretations of u th ra data suggest very different timescales of slab dehydration here we present new u th ra data from kamchatka along with a number of alternative models for production of radioactive disequilibrium variations in 226ra 230th and 231pa 235u activity ratios are best explained by crystal fractionation with host rock assimilation for a duration of less than c 6000 years the association of the largest 226ra excesses with high sr th in the most primitive lavas suggests that ra th fractionation is controlled by slab dehydration less than 10 ka ago we show that u th data can be explained by dynamic melting of a recently ka metasomatized mantle wedge dynamic melting of an oxidized source metasomatized several hundreds of thousands of years ago cannot produce significant 231pa excess because 238u 230th disequilibrium is inferred to be controlled by partial melting there is no requirement for multi stage slab dehydration commencing 150 ka we suggest that ra th disequilibria constrain the timing of slab dehydration whilst u th fractionation is dominated by partial melting at least at the kamchatka arc

tidal current dominated depositional environments in the central northern yellow sea as revealed by heavy mineral and grain size dispersals abstract this study confirms that the fine sand sized detritus in the central northern yellow sea is chiefly derived by the yellow river and explains the holocene yellow river dispersal system and the preservation of transgressive deposits by heavy mineral and grain size analyses micas which are sensitive to hydraulic sorting offer an effective key for identifying the dispersal of holocene yellow river derived fine grained sediments the heavy mineral compositional changes are attributed to hydraulic sorting by different marine dynamics and various mixing with additional local transgressive deposits of miscellaneous provenance the effects of hydraulic sorting are dominated by holocene tidal currents and by foreshore waves during the rapid transgression respectively the former results in obvious enrichments of garnet and common stable species such as ti oxides tourmaline and titanite whereas the latter seems more remarkable for the compositional changes with further depletion of epidote actinolite tremolite and zoisite clinozoisite the central yellow sea mud deposit and the alongshore muddy zone off the southern liaodong peninsula are opposite to each other as a pair of structurally symmetric coalesced terminal lobes far away from the high energy strait between cape chengshan and changsan got the haiyang subaqueous clinoform as another terminal lobe is revealed by a heavy mineral index which represents an extremity of deposits accumulated by the modern yellow river derived sediments off the southwestern shandong coast a small mud patch is revealed roughly corresponding to the central regime of the amphidromic system of principal m2 constituent the tidal current dominated depositional pattern proposed here emphasizes the importance of time velocity asymmetry of tidal currents such a dynamic mechanism is primarily responsible for the dispersal of the modern yellow river derived sediments when they are entrained from the bohai sea to the south yellow sea through the bohai strait and the strait between cape chengshan and changsan got

magma differentiation rates from 226ra 230th and the size and power output of magma chambers we present a mathematical model for the evolution of the 226ra 230th activity ratio during simultaneous fractional crystallization and ageing of magma the model is applied to published data for four volcanic suites that are independently known to have evolved by fractional crystallization these are tholeiitic basalt from ardoukoba djibouti morb from the east pacific rise alkali basalt to mugearite from vestmannaeyjar iceland and basaltic andesites from miyakejima izu bonin arc in all cases 226ra 230th correlates with indices of fractional crystallization such as th and the data fall close to model curves of constant fractional crystallization rate the best fit rates vary from 2 to 6 10 4 yr 1 consequently the time required to generate moderately evolved magmas f 0 7 is of the order of 500 to 1500 yrs and closed magma chambers will have lifetimes of 1700 to 5000 yrs these rates and timescales are argued to depend principally on the specific power output i e power output per unit volume of the magma chambers that are the sites of fractional crystallization equating the heat flux at the epr to the heat flux from the sub axial magma chamber that evolves at a rate of ca 3 10 4 yr 1 implies that the magma body is a sill of ca 100 m thickness a value which coincides with independent estimates from seismology the similarity of the four inferred differentiation rates suggests that the specific power output of shallow magma chambers in a range of tectonic settings covers a similarly narrow range of ca 10 to 50 mw km 3 their differentiation rates are some two orders of magnitude slower than that of the basaltic makaopuhi lava lake hawaii that cooled to the atmosphere this is consistent with the two orders of magnitude difference in heat flux between makaopuhi and the east pacific rise 226ra 230th data for magma suites related by fractional crystallization allow the magma differentiation rate to be estimated and from this the thermal budget of the magma chamber addressed and where an independent measurement of heat flux exists to place constraints on the size of the magma chamber such results have the potential to constrain the likely timescale and size of future eruptions of evolved magmas

melting dynamics beneath the tonga kermadec island arc inferred from 231pa 235u systematics measurement of 231pa 235u disequilibrium in tonga kermadec island arc lavas north of new zealand permits discrimination of the time scales and mechanisms of fluid addition and partial melting for tonga lavas the 231pa 235u ratios are less than 1 and can be explained by fluid addition from the subducting plate around 60 000 years ago in contrast the effects of partial melting overprinted those of fluid addition in the kermadec lavas resulting in 231pa 235u ratios greater than 1

u series disequilibria of trachyandesites from minor volcanic centers in the central andes abstract young trachyandesite lavas from minor volcanic centers in the central andes record the magma differentiation processes at the base of the lower continental crust here we report u series disequilibrium data for the historical lavas from the andagua valley in southern peru to define the time scale and processes of magmatism from melting in the mantle wedge to differentiation in the crust the andagua lavas show 230 th 238 u 231 pa 235 u and 226 ra 230 th above unity except for one more evolved lava with 230 th depletion likely owing to fractional crystallization of accessory minerals the 226 ra excess indicates that the time elapsed since magma emplacement and differentiation in the deep crust is within 8000 years based on the correlations of u series disequilibria with sio 2 content and ratios of incompatible elements we argue that the andagua lavas were produced by mixing of fresh mantle derived magma with felsic melt of earlier emplaced basalts in the deep crust because of the lack of sediment in the chile peru trench there is no direct link of recycled slabs with 230 th and 231 pa excesses in the andagua lavas instead 230 th and 231 pa excesses are better explained by in growth melting in the upper mantle followed by magma differentiation in the crust such processes also produced the 226 ra excess and the positive correlations among 226 ra 230 th sr th and ba th in the andagua lavas the time scale of mantle wedge melting should be close to the half life of 231 pa ca 33 ka while it takes less than a few thousand years for magma differentiation to form intermediate volcanic rocks at a convergent margin

crystal and magma residence at kilauea volcano hawaii 230th 226ra dating of the 1955 east rift eruption abstract previous estimates of crustal storage time of magmas at kilauea volcano hawaii range from a few years to a few thousand years leading to considerable uncertainty in the time scales of processes of magmatic storage and differentiation we present a new approach for determining minimum magma residence times which involves dating phenocrysts in a magma using 226ra 230th disequilibria and apply this approach to the early phase of the 1955 east rift eruption at kilauea when fractionation of ra from ba a proxy for initial ra in the crystals during crystal growth is considered along with the effects of inclusions in the minerals the data are consistent with co precipitation of plagioclase and clinopyroxene from a melt represented by the groundmass at a mean age of 1000 300 400 a unless a significant fraction 30 of the crystals are remnants from an earlier batch of evolved magma in the system these data constrain the minimum magmatic residence time to be 550 yr considerably longer than most previous estimates of storage time at kilauea as well as those for some other basaltic systems for the temperature interval of augite plagioclase growth in the early 1955 magma a maximum constant cooling rate of 0 1 c yr 1 10 5 c h is derived from the minimum magmatic residence time of 550 yr the total magma storage time would be 2500 yr if this cooling rate applied to the entire thermal history of the magma although a more complex cooling history where cooling rates were more rapid early in the storage history is permissive of a total residence time which is not much longer than 550 yr the disparate estimates of magma residence at kilauea may reflect the uncertainties in the methods of estimation in addition to true variations in storage time for different batches of magma more work is necessary in order to determine whether a long residence time is characteristic of rift zone lavas and or of kilauean lavas in general

thermoluminescence dating of the british coversand deposits abstract coversand deposits thought to be of lateglacial age are found in britain in north lincolnshire south west lancashire and central east anglia a comprehensive dating study of them using thermoluminescence tl techniques is currently underway in an attempt to link the british coversand deposits to the european coversand chronology initial results from four of the british coversand sites sampled are presented the 26 tl dates from 14 samples show that in lincolnshire aeolian deposition took place from 12 5 ka to i1 ka cessation of the initial sand deposition was synchronous with this in lancashire but sand deposition occurred significantly earlier in east anglia the upper layers of aeolian sand in lancashire are much younger and are attributed to holocene reworking on the basis of these dates lincolnshire and lancashire coversand deposition occurred at a similar time to the younger coversand ii whilst east anglian coversand deposition coincided with the younger coversand i phase in the european coversand chronology

evidence for a contribution from two mantle plumes to island arc lavas from northern tonga lavas from the islands of tafahi and niuatoputapu at the northern end of the active tonga kermadec arc in the southwest pacific were erupted at a convergent plate margin yet they can be shown to contain a contribution from two different mantle plumes high concentrations of nb relative to other high field strength elements in these lavas compared to other tonga lavas reflect an ocean island basalt component in the mantle wedge derived from the nearby samoa mantle plume pb isotope compositions indicate that most of the pb in these lavas is derived from the oceanic crust of the plume generated louisville seamount chain which is being subducted beneath the tonga arc these two plume components were thus introduced into the arc lavas in very different ways and provide insight into upper mantle dynamics and magma generation processes occurring in an active arc back arc system