magma mixing in boninite sequence of chichijima bonin islands abstract boninite from chichijima can be divided into two major types a porphyritic type phenocryst mode exceeds 1 and b aphyric type phenocryst mode is less than 1 both types are further subdivided by means of microphenocryst assemblages into five subtypes i olivine ol clinoenstatite cen bronzite brz ii ol brz iii brz iv brz pigeonite pig augite aug v cen brz there is a systematic correlation between these rock types and the bulk composition of aphyric boninites the most magnesian boninites are of type i and the least magnesian ones are of types iii and v type ii is slightly higher in cao than type i at a given feo mgo ratio type iv is distinct from the others for its high cao and low incompatible element contents at a given feo mgo ratio type iv cannot be derived from the others by crystallization differentiation compositional data for the other types of boninite define rectilinear compositional variations which lie on a tie line connecting fe rich bronzite and primitive boninite microphenocrysts in equilibrium with boninite magma are more magnesian so it would appear that the fe rich bronzite was in equilibrium not with boninite magma but with some evolved descendant e g bronzite andesite magma the variations of aphyric boninites are then best explained by a combination of mixing of primitive boninite and evolved descendants together with equilibrium crystallization processes in order to account for this combination of processes a density stratified open system magma chamber of the type proposed for oceanic ridge systems seems appropriate the upper part of the magma chamber is composed of differentiated bronzite andesite magma and the lower part consists of hot dense and primitive boninite magma crystallization and sporadic overturn and mixing give rise to the spectrum of magma types erupted at the surface

holocene vegetation and hydroclimatic dynamics in se lithuania implications from a multi proxy study of the cepkeliai bog abstract due to the scarcity of reliable palaeoecological and climatic proxy records from the eastern baltic covering the holocene there has been increased interest for multi proxy studies in the region to detect local to regional environmental changes the cepkeliai wetland complex se lithuania provides an uninterrupted holocene sediment sequence which is used here to reconstruct palaeoenvironmental history in the southern part of the eastern baltic high resolution lithological loi magnetic susceptibility palaeobotanical pollen plant macrofossil tree rings isotopic 14c and geochemical data were employed to reconstruct the peculiarities of vegetation and hydroclimate dynamics as well as to discuss results in a local to regional context temporal lags of environmental reactions to holocene climatic warming were recorded both in lithostratigraphical and biostratigraphical data thus confirming regional significance of this phenomenon a deep meso to eutrophic basin influenced by high surface runoff and surrounded by early boreal forest existed at the site until about 11 300 cal bp thereafter immigration of thermophylous taxa such as ulmus since 10 700 10 800 cal bp corylus since 10 400 10 300 cal bp tilia and quercus after 9500 cal bp as well as changes in the limnic environment point to an amelioration of the environmental situation i e to a predominance of a warm and dry climatic regime instability of palaeobotanical and lithological records suggests that environmental anomalies of the early holocene occurred at about 11 200 11 000 cal bp 10 600 10 300 cal bp and 8200 7800 cal bp these anomalies are thus contemporaneous with the pronounced climatic shifts i e dry and cool intervals known at the continental scale an expansion of picea was facilitated by the 8200 7800 cal bp climatic shift whereas early establishment of alnus glutinosa suggests an onset of a wet interval 10 200 cal bp which was previously unknown in the area the stepwise lowering of the water table noted at about 7300 7200 and 6600 6700 cal bp with the termination of the lake phase and change to telmatic environment after 6100 cal bp proves the positive reaction of the site to the dry climatic reversals of the middle holocene following the decay of the broad leaved thermophilous forest after 4500 cal bp a cold climatic reversal favored the formation of a diverse mosaic like vegetation which was then replaced by homogenous pinus picea vegetation after 3400 cal bp the climatic and site specific factors have been playing a leading role in the development of the site whereas signals of human interference remain negligible

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

os pb nd isotope and highly siderophile and lithophile trace element systematics of komatiitic rocks from the volotsk suite se baltic shield abstract highly siderophile element hse ru pd re os ir and pt abundances and re os isotope systematics in combination with lithophile trace element and sm nd and pb pb isotopic data are reported for komatiites and basalts from the volotsk suite in the se baltic fennoscandian shield the new sm nd and pb pb whole rock isochron ages of 2850 84 and 2861 150 ma respectively define the emplacement age of the volcanic sequence this age is statistically indistinguishable from the emplacement age of the lower mafic ultramafic sequences of the adjacent sumozero kenozero greenstone belt the re os internal isochron derived from the analysis of a whole rock b2 4 cumulate and three chromite separates yields an age of 2878 81 ma which is consistent with the ages obtained from the lithophile element isotopic systems the komatiites of the volotsk suite were derived from a mantle source characterized by depletions in highly incompatible lithophile trace elements with the magnitude of the time integrated lree depletion initial nd 2 7 0 2 typical of this period in the earth s mantle history yet this source had an initial 187os 0 4 0 4 and thus evolved with a time integrated re os that was identical within uncertainty to that of the carbonaceous chondritic reference in addition the calculated relative abundances of os ir ru and pt in the komatiite source are similar to chondrites and may be explained by late post core segregation accretion to the earth of materials with high absolute and chondritic relative abundances of hse or by re entrainment of the core if the former interpretation is correct the late accreted materials were well homogenized within the upper mantle by 2 9 ga like many other archean komatiites the studied rocks have radiogenic nd isotopic composition indicative of their long term relative depletions in highly incompatible lithophile trace elements yet the long term re os in their source remained near chondritic modeling indicates that the komatiite source must have experienced only a small degree of partial melting 2 3 and extraction of basaltic melts ca 150 ma prior to emplacement of the komatiitic sequences larger degrees of melting would otherwise have led to loss of a substantial fractionation of re in the source which would then have evolved to a subchondritic 187os 188os by the time of komatiite formation

pollen based quantitative reconstructions of holocene climate variability in nw romania abstract a modern analogue technique is applied to two high resolution pollen sequences from nw romania to provide the first quantitative evidence for winter summer and annual temperatures and for precipitation across the holocene in this region the pollen based climate reconstructions allow the identification of four main intervals i an early less stable period between 11 700 and 11 200 cal yr bp ii generally stable conditions between 11 200 and 8300 cal yr bp with winter and annual temperatures and precipitation higher than at present and summer temperatures about the same iii lower winter and annual temperatures and higher summer temperatures and precipitation between 8000 and 2400 cal yr bp iv warmer winter and annual temperatures and lower precipitation for the last 2400 years whereas summer temperatures became cooler at steregoiu and remained stable at preluca tiganului the pollen based climate reconstructions at the two sites show similar patterns in annual and winter temperatures and precipitation changes during the holocene but the trends appear to be less consistent for summer temperatures our pollen based reconstructions revealed several short term climatic oscillations during the holocene the strongest of which occur between 10 300 10 100 8300 8000 6800 6400 5100 4900 4000 3600 and 3200 3000 cal yr bp

holocene climate variability as reflected by mid european lake level fluctuations and its probable impact on prehistoric human settlements abstract a data set of 180 radiocarbon tree ring and archaeological dates obtained from sediment sequences of 26 lakes in the jura mountains the northern french pre alps and the swiss plateau was used to construct a holocene mid european lake level record the dates do not indicate a random distribution over the holocene but form clusters suggesting an alternation of lower and higher climatically driven lake level phases they provide evidence of a rather unstable holocene climate punctuated by 15 phases of higher lake level 11 250 11 050 10 300 10 000 9550 9150 8300 8050 7550 7250 6350 5900 5650 5200 4850 4800 4150 3950 3500 3100 2750 2350 1800 1700 1300 1100 750 650 cal bp and after 1394 ad a comparison of this mid european lake level record with the gisp2 polar circulation index pci record the north atlantic ice rafting debris ird events and the 14 c record suggests teleconnections in a complex cryosphere ocean atmosphere system correlations between the gisp2 pci the mid european lake level the north atlantic ird and the residual 14 c records suggest that changes in the solar activity played a major role in holocene climate oscillations over the north atlantic area

cold event at 8200 yr b p recorded in annually laminated lake sediments in eastern europe a quantitative annual mean temperature reconstruction from an annually laminated lake sediment sequence in estonia eastern europe shows a distinct cold period at 8400 8080 yr b p before a d 2000 the timing is consistent with that seen in the greenland ice core data and various high resolution records from western europe during maximal cooling at 8250 8150 yr b p the annual mean temperature in estonia was 2 0 c colder than prior to and 3 0 c colder than after the cooling the pollen stratigraphic and sedimentological data suggest especially cold and snowy winter conditions the duration and amplitude of the cold event agree with the modeled impact of a sudden freshening of the north atlantic surface water and subsequent perturbation of the thermohaline circulation provided that the cold event was caused by a pulse of freshwater from the melting laurentide ice sheet to the north atlantic the results indicate a strong teleconnection between the north atlantic oceanic forcing and the east european climate at least up to long 26 e mediated probably by the changing intensity of the zonal atmospheric circulation

late glacial and early holocene development of lakes in northeastern poland in view of plant macrofossil analyses the late glacial history of the development of seven extant or former now sediment filled lakes in northeastern poland was investigated based on high resolution plant macrofossil analyses and ams radiocarbon dating according to palaeobotanical data and ams dating the accumulation of organic sediments began during the bolling period the bottom sediments of all studied lakes contain plant macrofossils typical of late glacial vegetation such as betula nana dryas octopetala potamogeton filiformis and potamogeton alpinus the first plants to occur in the lakes were chara sp p filiformis and batrachium sp the late glacial sediments included rarely encountered macrofossils of sphagnum teres at the beginning of the holocene the water level in the studied northeastern polish lakes decreased significantly this was manifested in a change in the type of sediment transformation of small lakes into peatland in places and expansion of typical indicator plants of shallow waters

187os enriched domain in an archean mantle plume evidence from 2 8 ga komatiites of the kostomuksha greenstone belt nw baltic shield the re os data on archean komatiites from the kostomuksha greenstone belt in the baltic shield are presented this greenstone belt has been previously interpreted to represent a former oceanic plateau formed by the emplacement of an ancient plume head puchtel et al earth planet sci lett 155 1998 57 74 samples of flowtop breccia spinifextextured and cumulate komatiites and a chromite separate all collected from the core of a 300 m deep diamond drill hole yielded a re os isochron with an age of 2795 40 ma and an initial 187 os 188 os of 0 1117 0 0011 q 187 os 3 6 1 0 the high positive q 187 os t implies that the komatiites were derived from a mantle source with a time integrated suprachondritic re os ratio recycling of oceanic lithosphere to produce the enriched 187 os isotope signature is considered unlikely as 15 25 crustal component is required to be incorporated into the plume source as early as 3 5 4 3 ga such a substantial proportion of mafic material in the source would likely destroy the major and trace element characteristics of the komatiites our tentative interpretation is that the 187 os enrichment in the kostomuksha plume represents an outer core signature if confirmed by the ongoing pt os isotope studies the results would provide evidence for the existence of whole mantle convection in the late archean and might place constraints on the timing of core differentiation in the early earth fl 2001 elsevier science b v all rights reserved

lateglacial and early holocene palaeohydrological changes in the upper reaches of the ūla river an example from southeastern lithuania a reconstructed pattern of lateglacial and holocene hydrological changes in the area of the former lakes d bas pelesa and matara is presented the investigated basin is situated in southeastern lithuania beyond the marginal ridge of the weichselian glaciation on the margin of the sandy plain that is the watershed between the la and katra rivers pollen analysis radiocarbon dating loss on ignition measurements and gis based simulation of water level fluctuations have been applied in order to obtain new information on the development of the la katra watershed area the results of multiproxy investigations revealed the history of the development and extinction of d bas pelesa and matara lakes a single basin covering a large part of the study area had formed after the retreat of the continental ice sheet from southeastern lithuania this basin was divided into three separate lakes during the allerod interstadial as a result of a drop in water level blocking of the drainage by aeolian sediments aro

re os results from odp site 801 evidence for extensive re uptake during alteration of oceanic crust abstract re and os concentrations and os isotopic ratios were determined for composite samples prepared from volcanoclastics vcl and basaltic flows flo from jurassic oceanic crust ocean drilling program leg 185 site 801 in the western pacific with the aim of determining the effect of seafloor weathering on the re os budget a supercomposite sample prepared from a proportionate mixture of the various composite powders served to represent the average composition of the altered oceanic crust kelley k a plank t ludden j and staudigel h 2003 composition of altered oceanic crust at odp sites 801 and 1149 geochem geophys geosyst 4 6 8910 doi 10 1029 2002gc000435 re contents vary from 0 2 to 1 3 ng g 1 and from 2 2 to 3 1 ng g 1 in the vcl and flo composites respectively os contents vary from 0 005 to 0 047 ng g 1 in the vcl and from 0 008 to 0 027 ng g 1 in the flo composites the flo composites have much higher re os ratios and thus have more radiogenic os compositions 187os 188os 1 38 to 8 48 than the vcl composites 187os 188os 0 32 to 4 40 the vcl composite from the upper section of the crust shows evidence for substantial re loss and os uptake consistent with oxidative weathering processes however re uptake during weathering processes under more reducing conditions evident in the flo samples from throughout the section and to a lesser extent in the lower vcl samples more than compensates for this re loss in the upper vcl os concentrations were essentially unchanged by these reductive processes model age calculations suggest that re uptake continued for tens of millions of years after crust formation abundant secondary pyrite is found throughout the altered hole 801c crust in zones of restricted seawater flow and this may have accommodated an important part of the input re the re content of the supercomposite 2 2 ng g 1 is about 1 ng g 1 higher than would be expected on the basis of its yb content if the results from hole 801c are typical they suggest that the re concentration of at least the upper part of the oceanic crust may be nearly doubled during seafloor alteration such large extents of re uptake would have a significant effect on the oceanic re budget furthermore assuming that they survive passage through the subduction zone these elevated re contents would greatly decrease the proportion of subducted oceanic crust required in the source region to explain the radiogenic os compositions of many ocean island basalts

petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites abstract ji s salisbury m h and hanmer s 1993 petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites tectonophysics 222 195 226 in order to understand seismic anisotropy and its influence on the reflectivity of lower crustal fault zones we have undertaken experimental measurements of p wave velocities up to 600 mpa for 20 granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada the rocks whose composition ranges from ultramafic to felsic have experienced extremely large ductile strain it is found that the amphibolitic mylonites exhibit significant vp anisotropy 6 13 at 600 mpa whereas the pyroxene bearing granulite facies mylonites and the quartz or feldspathic tonalitic granitic and diatexitic mylonites are quasi isotropic in order to constrain interpretation of the measured vp properties microstructural analyses have been systematically performed in the samples studied it is shown that the interaction between feldspar and quartz or pyroxene and the absence of amphibole cause anisotropy in the granulite facies mylonites to be low the amphibolitic mylonites are strongly anisotropic because of a large volume fraction of hornblende with strong lpo 001 lineation and 100 foliation the anisotropy pattern in the amphibolitic mylonites is consistently orthorhombic vp x vp y vp z where x is parallel to stretching lineation y is normal to the lineation in the foliation plane and z is normal to the foliation plane but the anisotropy pattern in the mica rich mylonites is transversely isotropic vp x vp y vp z if sufficiently large volumes of the crust display such patterns this difference may be an important indicator for kinematic analysis in the middle to lower crust using modern seismic techniques the study demonstrates that while seismic reflectivity is strongly lithology controlled fabric induced anisotropy can both enhance and decrease seismic reflectivity hence the discontinuity of seismic reflections along ductile shear zones in the lower crust is not necessarily indicative of the discontinuity of the shear zones but may indicate changes in composition or metamorphic grade

a comparison of quartz c axis preferred orientations in experimentally deformed aplites and quartzites the effect of feldspar on the development of quartz c axis preferred orientations was investigated by experimentally deforming aplite 30 quartz and pure quartzite under equivalent conditions and comparing the quartz c axis patterns axial compression experiments were performed at temperatures of 800 and 900 c strain rates of 10 6 and 10 5s 1 and a confining pressure of 1500 mpa under these conditions both quartz and feldspar deform by dislocation creep with only minor syntectonic recrystallization the quartz c axis preferred orientations produced in the two rock types at equivalent deformation conditions and quartz grain strains were found to be essentially identical in pattern and strength these results indicate that the presence of feldspar has little or no effect on the operative slip systems in adjacent quartz grains in quartzo feldspathic rocks and that quartz c axis preferred orientation analyses can be performed on a wide range of crustal rock types

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

patterns of late pleistocene proglacial fluvial sedimentation in the se lithuanian plain abstract we investigated proglacial fluvial sedimentation processes in the se lithuanian plain by means of lithofacies analysis the main parameters on which interpretations were based were depositional structures of sandy sediments paleohydraulic parameters and grain size distribution the development of the se lithuanian plain illustrates the phenomenal shift of proglacial fluvial sedimentation from outwash plain to ice marginal river during the ice retreat of the last glaciation three facies assemblages of braidplain deposits and two facies assemblages of palaeovalley deposits were distinguished in the sandy plain by means of qualitative and quantitative sedimentological research this raises the question how the commonly accepted development of a se orientated outwash plain could turn into an almost perpendicular sw running ice marginal river the vertical and lateral transitions of outwash plain facies present a classical example of braidplain evolution on slightly inclined lowlands with well expressed proximal middle and distal parts mid and side channel point bar deposits of the proglacial valley point however which is unusual for proglacial fluvial systems to a meandering character of the ice marginal river river types of various scale show a change from braided into meandering in the proglacial subenvironment lithofacies analysis and paleohydraulic parameters show distinct differences of the hydrodynamic regime during the first stage of the sandy plain development from sedimentation on an outwash plain in a proglacial valley to sedimentation on a braidplain in a wide ice marginal valley the outwash system is characterized by a distinct downstream decrease in energy whereas the ice marginal river maintained most of its power and velocity

petrogenesis of volcanic rocks from saipan and rota mariana islands and implications for the evolution of nascent island arcs an 40 ar 39 ar age of 45 1 ma determined for lavas from northern saipan confirms that these high silica rhyolites erupted during the proto arc stage of volcanism in the izu bonin mariana system which is characterized elsewhere by eruption of boninitic lavas incompatible trace element concentrations and sr hf nd and pb isotope ratios for these rhyolites are transitional between those of c 48 ma boninitic lavas and post 38 ma first arc andesites and dacites from saipan and rota that have typical subduction related compositions these transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust a second stage of rayleigh fractionation in the upper crust is required by sio 2 concentrations that exceed 77 wt and near zero compatible element concentrations first arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first arc magmas themselves the mantle sources of the proto arc and first arc lavas from saipan and rota are similar to those of philippine back arc basin basalts based on nd and hf isotopic compositions the pb isotope compositions of these lavas are between those of pacific sea floor basalts and jurassic and younger cherty and clay rich sediments this contrasts with the boninitic proto arc volcanic rocks from guam and deep sea drilling project sites 458 and 459 that have pb isotope compositions similar to pacific basin basalts and volcaniclastic sediments the preferred explanation for the difference in the nature of proto arc volcanism between saipan and other fore arc locations is that the crust ceased extending 3 4 myr earlier beneath saipan this was caused by a change from mantle upwelling fore arc extension and shallow melting to an environment dominated by more normal mantle wedge convection stable crust and deeper melting

platinum group element fractionation in a komatiitic basalt lava lake precise pge os ir ru pt pd abundance data for bulk rock samples and mineral separates from a deep differentiated komatiitic basalt lava lake in the vetreny belt baltic shield were obtained using an icp ms id technique the composition of the erupted liquid mgo 5 15 is characterized by a highly fractionated pge pattern pd os n 5 48 with subchondritic os ir n 5 0 43 analysis of separated olivine and chromite revealed that ru os and ir were slightly compatible to moderately incompatible in olivine d 5 1 7 0 8 and were compatible with chromite d 5 100 150 platinum and pd were highly incompatible with olivine d 5 0 08 0 03 and moderately compatible with chromite d 5 1 6 3 3 bulk solid liquid partition coefficients for pges ni cr and cu estimated from the whole rock regressions were os 7 9 ir 6 6 ru 4 5 pt 0 53 pd 0 09 ni 6 2 cr 4 4 and cu 0 01 the highly incompatible behavior of cu and pd indicates that there was no separation of fenicu immiscible sulfide liquid during the differentiation of the lava lake comparison of the bulk partition coefficients with those calculated from the olivine and chromite compositions implies that the fractionating mineral assemblage included 10 2561 mass fraction of pyrrhotite the data obtained provide new constraints on the effects of fractional crystallization on the pge compositions of mafic ultramafic magmas copyright 2001 elsevier science ltd

distribution and origin of igneous rocks from the landward slopes of the mariana trench implications for its structure and evolution the landward slope of the mariana trench is composed largely of igneous rocks serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m seamountlike features on the trench slope break are the surface expression of serpentinite diapirs cumulate and massive gabbros are found several varieties of volcanic rocks are common including boninites altered and metamorphosed basalts andesites and dacites the chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism together with the gabbros these volcanic rocks represent what is probably a late eocene arc complex these rocks were probably the first volcanic products to result from the subduction of the pacific plate beneath the phillippine sea plate their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since eocene time most of this removal of material appears to have occurred during the early stages of subduction there are isolated occurrences on the landward slope of rock assemblages including alkalic basalts chert hyaloclastites upper cretaceous siliceous sediments and shallow water limestones these assemblages are very similar to rocks dredged from seamounts on the offshore flankmore of the trench and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion there has been episodic accretion of seamount fragments to the landward slope less

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

the effect of composition and strain on quartz fabric intensity in pebbles from a deformed conglomerate twenty eight quartz c axis fabrics from individual pebbles from a single outcrop of deformed conglomerate have been measured these pebbles vary in composition and have undergone different amounts of strain contrary to expectations based on published results of computer simulated deformation by intracrystalline slip of pure quartzites it is observed that fabrics are not stronger in the more highly deformed pebbles instead the fabric strength is related to the quartz content of the pebbles pure quartzites show the strongest fabrics as the percentage of non quartz minerals increases up to 25 the strength of the fabrics rapidly decreases though the influence of composition becomes less marked when impurities exceed 25 these observations can be explained if the deformation of the impure quartzites included a contribution from a non intracrystalline mechanism such as grain boundary sliding

the preboreal oscillation around the nordic seas terrestrial and lacustrine responses the occurrence of an early preboreal climatic cooling oscillation pbo in lacustrine and glacial records from northwest europe iceland and greenland is reviewed and documented the often subtle response of the proxy records to this oscillation in combination with its short duration make it difficult to detect owing to its chronostratigraphic position between the 10 000 9900 and 9600 9500 14 c plateaux c 11 300 11 150 calendar yr bp it is also difficult to 14 c date with precision we find that the vegetation response to the pbo varies between sites and regions in contrast to the pioneer vegetation in iceland and southern sweden the expanding birch pine forest in germany denmark was more susceptible to deteriorating growing conditions the combined lacustrine tree ring and glacial records imply that the pbo was characterised by cool and humid conditions throughout northwestern and central europe this is documented by vegetation changes decreased aquatic production increased soil erosion increased 2 h and 13 c content in tree rings readvances or stillstands of the ice sheet in norway and finland and ingression of brackish water into the baltic icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous preboreal period but not as extreme as during the younger dryas greenland records suggest that the early preboreal was characterised by ice readvances as an effect of cool climate and increased precipitation in relation to the younger dryas it was not until the end of the pbo that climate was warm enough to melt the land based ice sheet this preboreal oscillation found on both sides of the nordic seas is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the nordic seas which is implied by a simultaneous and distinct rise in the atmospheric 14 c 12 c ratio a slow down of the thermohaline circulation may tempor arily have pushed the polar front further south 1997 by john wiley sons ltd preboreal oscillation thermohaline circulation lacustrine records glacial records chronology

new insights into the last deglaciation of the south eastern flank of the scandinavian ice sheet abstract the existing glaciodynamic paradigm used to explain the dynamics morphogenesis and deglaciation of the last scandinavian ice sheet sis does not satisfactorily reflect all the nuances of ice sheet dynamics formation of particular forms of glacial relief and details of deglaciation significant problems are to be found in the explanations proposed for the processes of deglaciation therefore a new theory for the deglaciation of the last sis based on new geochronological geological and geomorphological data has been developed and is presented here it is suggested that along the south eastern perimeter of the sis with the exception of the salpausselka moraine ridges there is no evidence anywhere for re advances of the sis during deglaciation apart from the salpausselka moraine ridges the evidence for so called stadial or phasial recessional events is interpreted as the result of sis surges that occurred at different times and positions along the ice sheet margin this new theory also proposes that deglaciation of the south eastern flank of the sis was dominated by surface thinning rather than marginal retreat characteristic landforms such as kame terraces located on the distal slopes of recessional marginal ridges and plateau like glaciolacustrine kames were developed during deglaciation probably due to the interaction of active ice lobes surges and masses of dead ice that persisting beyond the ice margin

quartz preferred orientations within a deformed pebble conglomerate from new hampshire u s a abstract the orientations of quartz c axes within a deformed pebble conglomerate from piermont new hampshire have been measured and compared to finite strain estimates for both the pebbles and the surrounding matrix quartz grains within the relatively pure orthoquartzite pebbles show a strong c axis preferred orientation while quartz grains within the matrix show a random distribution of c axes comparisons of the preferred orientation of the pebbles with computer simulated fabrics of lister and hobbs 1980 yield estimates of the principal finite strains i 1 00 0 00 0 50 which are consistent with previous strain estimates rumble and oertel 1979 derived from the pebble shapes 1 22 0 13 0 48 the random c axis pattern within the matrix however is inconsistent with the strain estimates derived from fabric analysis of matrix phyllosilicates using the march model while it is believed that intracrystalline slip of quartz was as prevalent in the matrix as in the pebbles the presence of phyllosilicates in the matrix appears to have prevented the development of a quartz preferred orientation it is proposed that slip and or grain boundary sliding of phyllosilicates in the matrix may have been responsible for quartz grain rotations which were random with respect to quartz c axes

re os systematics of the ca 2 7 ga komatiites from alexo ontario canada abstract the re os isotope systematics of a suite of komatiitic flows from alexo township in the abitibi greenstone belt ontario were examined in order to characterize the partitioning behaviors of re and os during the generation and subsequent differentiation of komatiitic magma and also to determine the initial os isotopic composition of the late archean mantle source of these rocks this suite of komatiites was suitable for these objectives because of their high os concentrations 1 ppb and low 187 re 188 os ratios ranging between 0 4 and 2 4 the re os isotope results for whole rock komatiites and chromite separates yield a model 3 isochron age of 2762 76 ma this age is in general agreement with previously determined u pb ages of zircons from associated felsic units and also with sm nd and pb pb isochron ages of these flows this suite of alexo komatiites thus demonstrates relatively limited mobility of re and os at the whole rock scale even after large scale alteration and ambient lower greenschist facies metamorphism the initial os isotopic composition 187 os 188 os i 0 1080 0 0012 obtained from the regression is essentially chondritic os 0 1 1 0 our results therefore suggest an absence of any large scale deviation in initial os isotopic composition of the mantle source of these komatiites relative to the putative chondritic composition of the contemporaneous convective upper mantle for these komatiites os was moderately compatible with the mantle residue d os mantle melt 2 1 whereas re was incompatible d re 0 20 these d s are consistent with those previously estimated for other archean komatiites the apparent d os olivine liquid and d re olivine liquid in alexo komatiites are 1 7 and 0 66 respectively our results therefore suggest that os is slightly compatible whereas re is moderately incompatible in olivine or a coprecipitating phase during magmatic differentiation of komatiitic liquids the concentrations of re and os in alexo komatiites are also comparable to those reported for komatiites from other greenstone belts the apparent magmatic behaviors of both re and os in the whole rock komatiites indicate that the re os elemental systematics are primary features of these rocks the os isotopic results obtained for the alexo komatiites combined with those from the spatially associated munro township komatiites suggest a source for komatiites in the southern volcanic zone of the abitibi greenstone belt that was dominated by os with a chondritic isotopic composition also the os isotopic results combined with previous nd and pb isotopic results are consistent with derivation of the komatiites from either the contemporaneous convective upper mantle or a deep mantle source with isotopically similar characteristics these results imply that if the chondritic os isotopic composition of the mantle sources for the komatiites were set by a chondritic late veneer most late accretion and subsequent homogenization within the mantle must have occurred prior to their eruption at 2 7 ga

texture development in experimentally deformed two phase aggregates of calcite and anhydrite abstract fully quantitative textures have been measured from experimentally deformed two phase aggregates of calcite and anhydrite two sets of deformation experiments were performed firstly to investigate the development of microstructure and texture with composition samples with varying volume proportions of calcite and anhydrite were deformed to 20 strain at identical conditions secondly to study the texture development of the two materials with strain and explore the possible use of texture index as a strain gauge in polyphase materials pure calcite and pure anhydrite were deformed at the same conditions as the two phase rocks to strains of 6 12 and 20 the textures in both calcite and anhydrite were found to be similar but not identical to those reported in previous studies for pure samples our experimental investigations and measured textures show that the textures both of the pure materials and of the two phase rocks depend not only on strain but also on stress and grain size through their influence on the deformation mechanisms the dependence of texture index with change in deformation mechanism is particularly pronounced in the anhydrite component where the texture index decreases with grain size which decreases with anhydrite content the texture in the pure calcite and in the calcite component of the two phase rocks was influenced by a preexisting fabric the intensity of the texture in the deformed samples seems to correlate with the stress in the calcite rather than with strain

the link between strength of lattice preferred orientation second phase content and grain boundary migration a case study from the alpine fault zone new zealand abstract we analyse the microstructure and quartz lpos of 36 layers of varying composition from a several meter thick sequence of amphibolite facies metacherts and related mica garnet plagioclase quartz bearing schists from the central southern alps in the mylonite zone related to the alpine fault quartz contents vary from 10 to 100 and all of the lpo fabric skeletons are similar featuring an asymmetric single girdle of c axes inclined 30 away from the zy plane lpo strength is typically low at quartz contents

os isotope systematics of komatiitic basalts from the vetreny belt baltic shield evidence for a chondritic source of the 2 45 ga plume the vetreny belt in the southeast baltic shield is a large volcano sedimentary basin containing a 4 to 8 km thick sequence of basaltic to komatiitic lavas which were erupted 2 45 ga ago in a continental rift setting during the interaction of a mantle plume and the archean continental crust of the karelian granite greenstone terrane re os isotope data for olivine cumulate samples and chromite separates from victoria s lava lake and golets flow 3 define isochrons with ages of 2387 57 and 2432 34 ma respectively these ages are in good agreement with the previously reported and new nd pb and u pb zircon ages these data coupled with the evidence for immobile behavior of the re and os indicate that the re os system remained closed since the lava eruption the weighted average initial 187os values range between 0 43 0 10 for golets flow 3 and 0 07 0 13 for the lava lake a single chromite separate from golets flow 1 has a 187os t of 0 06 0 15 nd pb isotope and lithophile trace element data for the vetreny belt komatiitic basalts were used to monitor the effect of crustal contamination on the os isotope system and to estimate the initial os isotope composition of the vetreny plume source to be approximately chondritic with a 187os t of 0 9 this implies that the mantle source evolved with a long term nearly chondritic re os ratio the data provide further evidence that by the end of the late archean the earth s upper mantle was well homogenized with respect to the highly siderophile elements added during the accretion of a late veneer on a time scale of 1 ga

late glacial and early holocene lake level fluctuations in ne poland tracked by macro fossil pollen and diatom records abstract late glacial and early holocene vegetation history and lake level fluctuations in ne poland were described based on high resolution plant macrofossils analyses and records of pollen diatoms and radiocarbon dates the development of lake kojle perty began in the allerod gi 1c cooling during the younger dryas gs 1 led to the development of communities of heliophyte herbaceous plants palaeobotanical and lithostratigraphical records demonstrate that abrupt climate changes at approximately 11 600 and 9450 cal bp resulted in distinct changes in the vegetation and water level of lake kojle perty climate warming in the younger dryas holocene transition 11 600 cal bp caused the development of the terrestrial vegetation dominated by cold deciduous forest betula and appearance of thermophylious aquatic plants typha latifolia najas marina and ceratophyllum demersum a gradual decrease in the lake level since 11 200 cal bp is evident from the appearance of plants from shallow lake environments potamogeton natans and hippuris vulgaris the low water levels since 10 700 cal bp resulting from increased temperature and evaporation caused the development of peat on limnic sediments the dry and warm climatic phase with lowest lake level in the lakes of ne poland were coeval with the expansion of temperate deciduous forests corylus and ulmus that commenced at approximately 10 500 cal bp a distinctive increase in lake levels at approximately 9450 cal bp is suggested by the development of rush plants such as cladium mariscus and aquatic plants such as najas marina chara sp and nuphar lutea in the shallow parts of lake kojle perty the increased lake levels correspond with the expansion of alnus glutinosa in north east ne poland our reconstructions of the vegetation development and lake level changes align with results from other studies in the region

quartz c axis fabric differences between porphyroclasts and recrystallized grains abstract observed differences in c axis fabrics between quartz porphyroclasts and recrystallized quartz from the same quartzite samples suggest some changes in the fabric forming parameters during deformation the fabric patterns are consistent with a change in the incremental strain history from flattening to constriction during duplex development which is supported by the observed overprinting of structures in the field

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

ancient depleted mantle as a source of boninites in the izu bonin mariana arc evidence from os isotopes in cr spinel and magnetite abstract boninite is a volcanic rock derived from shallow melting of highly depleted hydrous mantle fluxed with water from subducted slabs the eruption of boninite early in the history of the izu bonin mariana ibm arc 48 45 ma suggests generation by melting of upper mantle material that was relatively unmodified by subducted components thus the boninite composition should largely reflect that of the sub arc mantle for better understanding of the mantle sources of nascent arc settings and the contributions of different components to arc melts we analyzed os isotope ratios 187os 188os of bulk rocks and mineral separates euhedral cr spinel from boninites and euhedral cr spinel magnetite mixtures from tholeiites younger than 45 ma that erupted after boninites from the bonin islands and guam the age corrected initial os isotope ratios of the whole rock samples 0 1179 0 2050 were more radiogenic and variable than those of the mineral separates possibly because of contamination with crustal materials during magma ascent or alteration after emplacement the age corrected os isotope ratios of euhedral cr spinel in boninite from the bonin islands 0 1187 0 1254 and from guam 0 1220 0 1269 are unradiogenic relative to primitive mantle and those of the cr spinel magnetite mixtures from the tholeiites from the bonin islands are similar to or slightly more radiogenic 0 1224 0 1382 the most depleted os isotope ratio of the cr spinel from boninite yielded a model re depletion trd age of 1 4 ga suggesting that the mantle source of the boninite experienced melt extraction prior to 1 4 ga the source of the boninites is interpreted to be mostly highly depleted mantle with a small contribution of slab flux arising from altered oceanic crust that has radiogenic os components with or without contributions from components with relatively unradiogenic os such as volcaniclastics of oceanic island basalt affiliation or very young mid ocean ridge basalt

grain growth kinetics of quartz and calcite aggregates grain growth rates for novaculite flint and jasper have been determined between 1000 and 800 c and 15 to 2 kb water pressure a pre treatment established an equilibrium microstructure and a nearly uniform mean grain diameter bar d 0 at 1000 c and 15 kb the grain growth data are linear on a log bar d bar d 0 vs log time plot and there is no significant difference between the three samples although the jasper contained 5 10 vol iron oxide at lower pressures and temperatures the growth curves are approximately parallel to the 1000 c and 15 kb data but growth is slower and that for jasper is slower than for the other two the growth rate is strongly dependent on the solubility of quartz in the intergranular fluid and the activation energy is about 11 kcal mole the data are used to estimate the growth rate of quartz at metamorphic conditions and to help evaluate the conditions where growth of dynamically recrystallized grains may be significant the growth rate of calcite grains s

new insights into the origin of o hf os isotope signatures in arc lavas from tonga kermadec abstract o hf and os isotope data are presented for lavas from the highly depleted tonga kermadec arc o isotope values overlap with those of morb limiting the amount of interaction with the arc crust 18 o does not increase northwards as would be expected from the 4 fold increase in subduction rate if slab derived fluids had high 18 o 16 o ratios thus the overall northward decrease in hfse concentrations likely reflects depletion due to prior melt extraction not increasing extents of melting hf isotopes are strongly negatively correlated with be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require hf to be fluid mobile with the exception of a boninite from the north tongan trench the northern tonga lavas do not overlap the hf isotope composition of either the samoan plume or the subducting louisville volcaniclastic sediments thus the pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the louisville volcaniclastics with minimal mobilisation of hf this suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments 187 os 188 os ranges from 0 1275 to 0 4731 and the higher os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust conversely the lowest os ratios are subchondritic and indicate that transfer of radiogenic os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge remarkably the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle derived magmas with minimal interaction with the arc crust

present day events and the evaluation of holocene palaeoclimatic proxy data abstract reconstruction of holocene climatic changes should be based on correlation of proxy data representing various sediment facies and ecosystem characteristics in addition to long term fluctuations in such data various extreme events such as downpours floods and slope failures whose type and frequency fluctuate in time have also been recorded events may reflect global regional or local changes examples from poland are used to illustrate general principles the observation of present day processes and their erosional or deposition effects linked with proxy indicators from the past should help in interpretation of environmental changes in the holocene and to inferences about the causative mechanisms

quartz deformation mechanisms in metasediments from prins karls forland svalbard abstract quartzites and phyllites of presumed precambrian cambrian age in western spitsbergen were deformed and metamorphosed at middle to lower greenschist facies conditions they exhibit a wide variation in quartz content with less variable quartz grain sizes quartz microstructures and lattice orientations in these rocks are used to infer which deformation mechanisms were dominant in the different lithologies three variable factors in the quartzites and phyllites may have influenced the way in which quartz deformed these are 1 1 quartz content 2 2 average quartz grain size 3 3 the distribution of accessory phases particularly the micaceous minerals between the quartz grains generally in a rock with a high quartz content a coarse average quartz grain size and or poorly distributed accessory phases quartz exhibits microstructures and lattice orientations which suggest that intracrystalline mechanisms dominated during deformation on the other hand in a rock with a low quartz content a fine average quartz grain size and or evenly distributed accessory phases quartz exhibits microstructures and lattice orientations which suggest that it deformed by means of intercrystalline mechanisms

new osmium isotope evidence for intracrustal recycling of crustal domains with discrete ages new 187os 188os ratios of quaternary mount adams volcanic rocks from the cascade arc in southern washington vary by 300 187 os 188 os 5 0 165 0 564 and fall into high 0 319 and low 0 166 to 0 281 groups of 187 os 188 os ratios that are sub stantially more radiogenic than mantle values these os isotope compositions and groupings are interpreted to reflect recycling of discrete intracrustal domains with high 187os 188os ratios but dif fering ages thus recording the process of crustal hybridization and homogenization os isotope compositions provide new constraints on amounts of intracrustal recycling in young subduction zone en vironments that reflect the magmatic history of the arc sr nd hf and pb isotope variations in this young mafic arc complex are too small to allow such constraints

natural transformation of the romoty paleolake ne poland during the late glacial and holocene based on isotopic pollen cladoceran and geochemical data abstract romoty is the name of a sediment profile from the overgrown lake located in elk lakeland ne poland the sediment core which is 8 4 m in length documents the development of the reservoir since the melting of dead ice and formation of the lake basin i e before 13 000 years bp until it became overgrown with plants the characteristics of the sediments changed from mineral deposits on the bottom silty sands to biogenic deposits consisting of dark olive calcareous detritus gyttja and light calcareous gyttja with increasing carbonate content and to peat in the upper part based on the data chemical stable isotopes subfossil cladocera remains pollen we have distinguished three main stages of the evolution of romoty paleolake the first stage includes the formation of the lake basin in the late glacial and early holocene preboreal the second stage is characterized by the harmonic natural development of the romoty paleolake from the boreal to subboreal this stage coincides with a period of climatic warming and biogenic sediments accumulated in this reservoir at that time the third stage concerns the disappearance of the lake in the late holocene subatlantic the lake conditions declined and the reservoir completely filled with sediments and has transformed into a mire the characterization of each stage involves a description of environmental conditions occurring in the romoty paleolake particularly the cladocera community the chemical composition the temperature and trophy of the water the water level the nature of bottom sediments and the plants occurring in the reservoir and its surroundings the lowest water temperature occurred in the late glacial and preboreal periods since the boreal period gradual warming is observed and is expressed through a positive trend in both 18 o and 13 c and changes in both cladocera and plant compositions the lake was deepest at the beginning of the holocene as a result of dead ice melting supply of melt water lowering of the lake bottom and or increased precipitation change in atmospheric circulation the water level fell during the atlantic period additionally the cladoceran and non pollen palynomorph data indicate low trophy followed by an increase in the mesotrophic state human impact was documented only by a low occurrence of pollen grains of cerealia in the youngest sediments subatlantic period suggesting a low penetration by human groups of the area near the romoty paleolake

textures and c axis orientations of deformed quartz crystals from porphyric dikes of the alpine root zone western alps deformation textures and c axis preferred orientations of quartz phenocrysts from porphyric dikes of the alpine root zone have been formed under conditions of greenschist to low amphibolite facies it is shown that the deformation textures as shear planes and kink bands boudins grain boundary sutures subgrains coarse and fine recrystallized grains deformation lamellae and fractures are developed in a chronological sequence during first increasing and later decreasing temperatures

influence of micaceous impurity on dynamically recrystallized quartz c axis fabric in l s tectonites from the singhbhum shear zone and its footwall eastern india when temperature and strain rate remain constant the quartz c axis fabric in deformed pure quartz aggregates is largely dependent on deformation kinematics asymmetry of the fabric e g in type i asymmetric crossed girdle pattern in natural quartz tectonites simulated fabric or experimentally deformed quartz aggregates is generally related to sense of vorticity for a non coaxial flow natural quartz tectonites however often contain micaceous impurities measurement on a sample of 59 quartz tectonites with mesoscopic l s fabric and representing low ttm deformation under non coaxial flow from the singhbhum shear zone and dhanjori quartzites eastern india provides the basic data to quantitatively assess the influence of mica on i asymmetry of quartx c axis fabric and ii degree of crystallographic preferred orientation i e fabric intensity taking c axes of dynamically recrystallized quartz grains as a fabric element a fabric intensity parameter is defined as the ratio of the greatest eigenvalue to the least eigenvalue of the orientation tensor matrix corresponding to c axis orientations in each measured specimen the modal percent of mica in the sample varies from 2 to 35 that of recrystallized quartz grains as opposed to relict clasts from 45 to 98 the asymmetry of the fabrics in the above sample measured either as the am statistic or as the angle between the central segment of the fabric skeleton and direction of mineral elongation lineation is independent of mica content correlation regression analysis of the variables and demonstrate a negative correlation between micaceous impurity and the fabric intensity parameter the regression equation is of the form 0 13 1 11 0 617

comparison of quartz microfabric with strain in recrystallized quartzite abstract the relationship between quartz c axis microfabric and strain is examined in six specimens of recrystallized quartzite conglomerate in which strain was measured using pebble shapes four rocks subjected to plane strain display a direct relationship between the strength of preferred orientation and the strain intensity the c axis distributions in these rocks as well as a rock subjected to moderate extensional strain are crossed girdles with maxima near the intermediate principal strain axis and connecting girdles at acute angles to the direction of maximum shortening a rock subjected to moderate flattening strain has several maxima clustered near the direction of maximum shortening and a weak connecting girdle through the intermediate principal strain axis these results are generally similar to those of other studies comparing strain and tectonite fabrics and also with experimental and computer simulation studies of fabrics the degree of preferred orientation is related to total strain and therefore microfabrics in quartzites may be cautiously interpreted as qualitative indicators of strain intensity uncertainties are greater however for correlations of fabric patterns with shapes of the strain ellipsoid an observed increase in recrystallized grain sizes with increasing strain suggests that flow stress was lower in the more strained rocks

geology and geochemistry of early arc volcanic rocks from guam the island of guam is located at the southern end of the mariana fore arc and was the site of early arc volcanism between 43 and 32 m y ago two volcanic units were erupted during the early evolution of guam the late middle eocene facpi formation and the late eocene to early oligocene alutom formation the facpi formation is composed largely of interbedded boninite series pillow lavas pillow breccias and dikes although arc tholeiite series rocks cap the formation in some areas the alutom formation is composed of interbedded volcanic breccias tuffaceous sandstones lava flows and sills calc alkaline arc tholeiite and boninite series compositions are all found in the alutom formation primitive boninite series rocks are relatively high in sio 2 mgo and ni and low in a1 2 o 3 and tio 2 contents compared to basaltic rocks from the arc tholeiite series additionally they have low ti zr ratios and high k zr rb zr and ba zr ratios although no calc alkaline basalts are found in the early arc formations of guam basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc alkaline series among the analyzed samples here are a few with anomalously high y and rare earth element ree contents chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase clinopyroxene and orthopyroxene the arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase clinopyroxene orthopyroxene and magnetite the composition of the least silicic calc alkaline series andesite can be modeled by fractional crystallization of olivine clinopyroxene plagioclase and magnetite from a transitional basalt derivation of more silicic calc alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende all basalts from the early volcanic series of guam were erupted in an arc setting the boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels whereas the arc tholeiite and calc alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels we suggest that because the calc alkaline series parent basalts most likely were richer in incompatible high field strength hfs elements and sio 2 than were the tholeiitic parents they were generated by lower degrees of melting

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research