magma mixing in the aleutian arc evidence from cognate inclusions and composite xenoliths abstract the complexity of igneous processes in the aleutian calc alkaline magma series can be inferred from study of xenolithic fragments composite xenoliths and cognate inclusions provide direct evidence for magma magma and wall rock magma mixing processes using distributions of cr in clinopyroxene compositional endmembers involved in mixing are identified within the xenoliths the basaltic mixing endmember is more mafic than calc alkaline lavas in the arc magma mixing and wall rock assimilation within calc alkaline basaltic to andesitic magmas is identified in phenocrystic assemblages as well as in xenoliths and appears to be a widespread phenomenon in aleutian calc alkaline magmas

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

constraints on magma genesis behind the neogene cascade arc evidence from major and trace element variation of high alumina and tholeiitic volcanics of the bear creek area lava flows and dikes from the bear creek area within the neogene blue mountains province bmp were erupted in an extensional tectonic environment major and trace element variations in the bear creek suite constrain models for the petrogenesis of these rocks bear creek basalts basaltic andesites and andesites fall into two compositional groupings high alumina and tholeiitic the most primitive flow is high alumina in composition with an mg 0 69 various phase equilibria tests suggest that magma of this composition may be a hybrid of an eclogite derived melt and a peridotite contaminant or it could be a melt derived wholly from a peridotite source large ion lithophile element enrichment and high field strength element depletion especially nb in this sample are consistent with its derivation from a fluid metasomatized peridotite source an alternative source is required for three bear creek tholeiites and high alumina basalts habs that differ from the other bear creek samples primarily in having larger nb contents relative to other incompatible trace elements this source can be modeled as partial melts of a peridotite which has been contaminated by a pelagic sediment presumably from the subducted slab least squares mixing calculations with high pressure mineral phases from peridotites indicate that high pressure fractionation may have generated the more primitive high alumina types in the bear creek suite however using the computer program silmin ghiorso 1985 to semiquantitatively model shallow magmatic processes demonstrates that assimilation of greywacke and rhyolite as a partial melt of the greywacke and fractional crystallization afc at 1 kbar pressure better explain the major element variation in the high alumina types the tholeiites appear to be derived from high alumina parents via fractional crystallization at shallower levels in the crust semiquantitative modeling using trace element distributions supports a model in which the hab suite was derived from peridotite and underwent afc in the upper crust and the tholeiites were derived via rayleigh fractionation from hab parents the trace element modeling also indicates that the range of trace element abundances cannot be explained by mixing of melts derived from eclogite and peridotite therefore a model in which dehydration of the farallon plate metasomatizes mantle wedge peridotite and where pelagic sediment is incorporated into the peridotite which subsequently melt to yield magmas that are modified by contamination and fractionation in the upper crust explains all of the geochemical variation in the bear creek area rocks this model may in part be applicable to the origin of the picture gorge and slide creek volcanic sequences in the bmp

a petrologic re investigation of the adak volcanic center central aleutian arc alaska the adak volcanic center is located in the central part of the aleutian arc and consists of three main volcanic vents andrew bay volcano the oldest center has been mostly removed by erosion the next youngest vent mount adagdak was built in three major volcanic stages whereas mount moffett the largest volcanic edifice consists of a main cone and a parasitic cone each with several magmatic phases adak is unique compared to other modern aleutian volcanic centers in that it contains two xenolith suites conrad and kay 1984 debari et al 1987 one suite consisting predominantly of mafic xenoliths occurs on mount moffett whereas an assemblage of ultramafic and mafic xenoliths is found on mount adagdak lavas erupted at adak span the compositional range from 48 4 to 65 0 wt sio2 and are characterized by significant variations in al2o3 mgo sr ni and cr on harker diagrams this variability produces compositional trends with significant scatter the adak suite has total ree contents that vary from 32 to 154 ppm but do not correlate systematically with silica layb n ratios range from 2 41 to 21 72 with the majority of lavas between 2 41 and 6 06 on process identification diagrams the adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other aleutian centers measured isotopic ranges are large and nearly equal to those for the entire arc although they span similar silica ranges subtle geochemical and isotopic differences distinguish the different volcanic vents of adak on mount moffett a geochemically and isotopically distinct group of andesites 55 5 57 9 sio2 the mafic andesites occur on its ne flank these lavas have elevated mgo ni and cr but are depleted in al2o3 relative to other mount moffett andesites with similar silica they also have more heterogeneous ree abundances and isotopic ratios than most of the other andesites significant compositional differences exist between adak and the other volcanic centers of the central aleutian arc although these differences are characteristic of all geochemical systems they are greatest for major and rare earth elements and isotopic ratios the lack of coherent relationships on major and trace element harker diagrams the isotopic variability as well as the steeply sloping trends on ree process identification diagrams suggest that the adak volcanic suite was not formed predominantly by closed system crystal fractionation but must be the product of a complex open system process es the significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite ree data are also suggestive of a role for magma mixing such a complex petrologic evolution is consistent with an immature lithospheric plumbing system based on ree systematics the xenolith suites of adak cannot as previously proposed be related to the host lavas or the rest of the adak suite through crystal fractionation schemes rather they are probably accidental fragments derived from various depths along lithospheric conduits in light of their relation to xenolith bearing units the mafic andesites of adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock they are therefore characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas because of the close proximity of the vents and the subtle compositional differences between their lavas the adak volcanic center was probably supplied by a single deep lithospheric plumbing system that fed separate crustal magma chambers the absence of historic volcanic activity on adak suggests this plumbing system was abandoned before complete conduit development this decline in magmatism may reflect a re adjustment of volcano spacing within this part of the aleutian arc

strike slip earthquakes on quasi vertical transcurrent faults inferences for general scaling relations the recent occurrence of several large strike slip earthquakes provides the opportunity to review and complement available data on the scaling of seismic moment mo with length of rupture l for large earthquakes depending on their tectonic setting and mechanism for strike slip earthquakes on quasi vertical transcurrent faults the mo versus l relation has a significant change of slope around mo 0 6 0 8 1020 n m and for larger earthquakes mo scales linearly with l this is compatible with models where slip is controlled by the width of the fault also it appears to be easier to categorize large earthquakes by their mechanism strike slip on vertical transcurrent fault versus pure thrust normal than their tectonic setting interplate intraplate

mapping the rupture process of moderate earthquakes by inverting accelerograms we present a waveform inversion method that uses recordings of small events as green s functions to map the rupture growth of moderate earthquakes the method fits p and s waveforms from many stations simultaneously in an iterative procedure to estimate the subevent rupture time and amplitude relative to the green s function event we invert the accelerograms written by two moderate parkfield earthquakes using smaller events as green s functions the first earthquake m 4 6 occurred on november 14 1993 at a depth of 11 km under middle mountain in the assumed preparation zone for the next parkfield main shock the second earthquake m 4 7 occurred on december 20 1994 some 6 km to the southeast at a depth of 9 km on a section of the san andreas fault with no previous microseismicity and little inferred coseismic slip in the 1966 parkfield earthquake the inversion results are strikingly different for the two events the average stress release in the 1993 event was 50 bars distributed over a geometrically complex area of 0 9 km2 the average stress release in the 1994 event was only 6 bars distributed over a roughly elliptical area of 20 km2 the ruptures of both events appear to grow spasmodically into relatively complex shapes the inversion only constrains the ruptures to grow more slowly than the s wave velocity but does not use smoothness constraints

radiative dynamical consequences of dry tongues in the tropical troposphere abstract dry layers are frequently observed in atmospheric soundings from the climatologically humid western pacific warm pool region some 2400 soundings from the toga coare field program were objectively examined for humidity drops layers in which relative humidity decreases rapidly with height indicative of the bases of dry layers these occur throughout the lower and middle troposphere with frequency peaks near 550 the 0 c level 800 and 950 mb a composite constructed from these sounding data indicates the following almost all dry layers are too dry and not warm enough to be interpreted as conservative vertical displacements rather they apparently consist of filaments or tongues of low moist static energy air advected into the column often from the subtropics dry tongues are anomalously virtually warm near their bases with a slight cool layer below that is they sit atop sharp stable layers or inversions the authors hypothesize that radiation is responsible for the thermal structure o

geochemical and strontium isotopic characteristics of parental aleutian arc magmas evidence from the basaltic lavas of atka eighteen flows from a basal stratigraphic sequence on the aleutian island of atka were analyzed for major elements trace elements and initial 87sr 86sr ratios petrographically these lavas contain abundant plagioclase 24 45 and lesser amounts of olivine 7 magnetite and clinopyroxene phenocrysts compositionally the lavas are high alumina 20wt basalts 48 51 wt sio2 with low tio2 1 and mgo 5 within the section compositional variations for all major elements are quite small while mgo content correlates with olivine phenocryst contents no such relationship exists between the other oxides and phenocryst content these lavas are characterized by 8 10 ppm rb high sr 610 669 ppm 308 348 ppm ba and very constant zr 23 29 ppm and sc 23 29 ppm abundances ni and cr display extremely large compositional ranges 12 118 ppm and 12 213 ppm respectively no correlation exists between trace element concentrations and phenocryst contents strontium isotopic ratios show a small but significant range 0 70314 0 70345 and are slightly elevated with respect to typical morb no systematic correlation between stratigraphic position and petrography or geochemistry is evident ree abundances measured on six samples are lree enriched la yb n 2 20 2 81 and display similar chondrite normalized patterns one sample has a slight positive eu anomaly but the other lavas do not compared to other aleutian basalts of similar silica content these lavas are less lree enriched and have lower overall abundances the geochemical characteristics of these basalts suggest they represent true liquid compositions despite their highly porphyritic nature published phase relations indicate fractionation of a more mgo rich magma could not have produced these lavas the high al2o3 and low mgo and compatible element abundances suggest a predominantly oceanic crustal source for parental high alumina basalts

a new spectral ratio method using narrow band coda envelopes evidence for non self similarity in the hector mine sequence 1 the use of local and regional s wave coda is shown to provide stable amplitude ratios that better constrains source differences between event pairs we first compared amplitude ratio performance between local and near regional s and coda waves in the san francisco bay region for moderate sized events then applied the coda spectral ratio method to the 1999 hector mine mainshock and its larger aftershocks we find 1 average amplitude ratio standard deviations using coda are 0 05 to 0 12 roughly a factor of 3 smaller than direct s waves for 0 2 f 15 0 hz 2 coda spectral ratios for the mw 7 0 hector mine earthquake and its aftershocks show a clear departure from self similarity consistent with other studies using the same datasets and 3 event pairs green s function and target events can be separated by 25 km for coda amplitudes without any appreciable degradation in sharp contrast to direct waves

regional difference in scaling laws for large earthquakes and its tectonic implication we compiled 67 large earthquakes which occurred at and around plate boundaries for the last 140 yrs and classified them into four groups interplate strike slip events intraplate strike slip events underthrust events at island arc subduction zones and underthrust events at continental margin subduction zones for each group of earthquakes we examined relations between seismic moment m 0 fault length l fault width w and average fault slip d and found the following scaling laws in the case of interplate strike slip events the well known l cubed dependence of seismic moment breaks down when l exceeds 30 km because the extent of the seismogenic zone is limited in depth 12 km for large events l 30 km d and m 0 increase with l as d overline delta tau l mu left alpha l beta right and m 0 overline delta tau w l 2 left alpha l beta right respectively where the mean fault width bar w is 12 km and the mean stress drop overline delta tau is 1 8 mpa here and are structural parameters for intraplate strike slip events we obtained nearly the same relations except for significantly higher stress drop 3 1 mpa the difference in stress drop between interplate and intraplate events may be ascribed to the difference in stress accumulation rates and thus the recurrence time of earthquakes in the case of underthrust events at island arc subduction zones we also found the saturation of fault width bar w 120km and the breakaway from the l cubed dependence of m 0 for events larger than l 200 km if we consider the average dip angle of plate boundaries at island arc subduction zones to be 20 30 this indicates that the extent of the seismogenic zone in depth is limited to 40 60 km in the case of continental margin subduction zones on the other hand we could not find the saturation of fault width nor the breakaway from the l cubed dependence of m 0 from the analysis of the present data set w 200 km l 1000 km for sufficiently large earthquakes in general the downward rupture growth is limited to a certain depth due to the existence of a ductile unstressed region which extends under the brittle seismogenic zone since the brittle ductile transition occurs at 300 400 c the difference in the lower limit of the seismogenic zones between tectonically different regions may be attributed to the difference in thermal state there

230 th 238 u disequilibrium in island arcs evidence from the aleutians and the marianas complex models for magma generation in island arcs have recently been proposed which require varying contributions from three possible sources 1 the mantle 2 subducted altered oceanic crust and 3 sediments see for example refs 1 6 these models involve partial melting of metasomatized mantle the metasomatizing liquids derive from the subducted slab and are large ion lithophile lil element bearing fluids or are themselves partial melts further modification of the chemical composition of the lavas may occur by processes such as shallow level fractional crystallization because the th u ratio is quite different for each of the three possible magma sources mentioned above and as th and u may behave differently during fractionation processes particularly if volatile transport is involved studies of 230 th 238 u disequilibrium have great potential in investigations of arc lavas our data given here imply that the th u ratio in the sources of mariana and aleutian island arc lavas is similar to ocean ridge basalt sources although there is a hint of a somewhat lower th u component in the aleutian source the data also confirm and extend the observations by gill and allegre and condomines that oceanic arc lavas are distinguished from magmas erupted in virtually all other volcanic environments by activity ratios 230 th 238 u 1 implying relative enrichment of uranium

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

a study of thermals in cumulus clouds air motions in the thermals contained within shallow florida cumulus clouds were observed to be similar to the circulation observed in laboratory thermals there was outward flow in the updraughts of individual thermals at most levels and there were usually downdraughts observed at the edges of the updraught or of the cloud widespread inward flow towards the centre of the cloud and a narrow but strong updraught was occasionally observed reminiscent of the tail found at the rear of the laboratory thermals a region of reduced liquid water content was frequently observed in the centre of several thermals where the updraught and horizontal 1d divergence were strongest and complete holes were observed on two occasions although horizontal wind shear was generally weak it was significant in a few cases causing the flow pattern in the cloud to be asymmetric ascending regions of cloud with high values of liquid water content cloud cores were commonly observed at all altitudes but generally the percentage of clouds measured with high liquid water content decreased with altitude the observations of airflow and liquid water content structure in warm cumulus clouds described in this paper are consistent with the schematic model of a thermal where a core of high liquid water content survives for several kilometres above cloud base but erodes as it ascends copyright 2005 royal meteorological society

thermodynamic variability within the convective boundary layer due to horizontal convective rolls abstract data from the convection and precipitation electrification cape experiment conducted during the summer of 1991 are used to examine and quantify the horizontal variability of temperature and moisture within the convective boundary layer cbl potential temperature variations were only about 0 5 k while variations in water vapor mixing ratio values of 1 5 2 5 g kg 1 were observed throughout the cbl using radar aircraft and sounding data it is shown that horizontal convective rolls are the likely cause of these variabilities the enhanced moisture occurred within the roll updraft regions thus rolls were transporting moist air from the surface upward the observed cloud base heights obtained from cloud photogrammetry were produced from the highest moisture values within the roll updraft regions since the roll ascending branches contained moisture values that were most representative of the observed cloud base heights it is likely that measurements from within the roll updrafts would prov

mantle flow through the northern cordilleran slab window revealed by volcanic geochemistry the northern cordilleran slab window formed beneath western canada concurrently with the opening of the californian slab window beneath the southwestern united states beginning in late oligocene miocene time a database of 3530 analyses from miocene holocene volcanoes along a 3500 km long transect from the northern cascade arc to the aleutian arc was used to investigate mantle conditions in the northern cordilleran slab window using geochemical ratios sensitive to tectonic affi nity such as nb zr we show that typical volcanic arc compositions in the cascade and aleutian systems derived from subduction hydrated mantle are separated by an extensive volcanic fi eld with intraplate compositions derived from relatively anhydrous mantle this chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the northern cordilleran slab window we suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge which had developed during extensive early cenozoic arc and backarc volcanism in western canada high heat fl ow throughout the western canadian cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window induced mantle upwelling

initial rupture of earthquakes in the 1995 ridgecrest california sequence close examination of the p waves from earthquakes ranging in size across several orders of magnitude shows that the shape of the initiation of the velocity waveforms is independent of the magnitude of the earthquake a model in which earthquakes of all sizes have similar rupture initiation can explain the data this suggests that it is difficult to estimate the eventual size of an earthquake from the initial portion of the waveform previously reported curvature seen in the beginning of some velocity waveforms can be largely explained as the effect of anelastic attenuation thus there is little evidence for a departure from models of simple rupture initiation that grow dynamically from a small region the results of this study indicate that any precursory radiation at seismic frequencies must emanate from a source region no larger than the equivalent of a m0 5 event i e a characteristic length of 10 m the size of the nucleation region for magnitude 0 to 5 earthquakes thus is not resolvable with the standard seismic instrumentation deployed in california

petrologic constraints on the spatial distribution of crustal magma chambers atka volcanic center central aleutian arc the atka volcanic center the largest modern central aleutian magmatic complex covers 360 km2 with an estimated eruptive volume of 200 km3 the oldest exposed rocks the old harbor series are a suite of basaltic andesite to dacite flows pyroclastic rocks and lahars of presumed tertiary age the youngest phase of activity on atka probably began 1 2 ma ago near the center of the present volcanic field and consists of basaltic and basaltic andesite flows which produced a large shield volcano atka volcano subsequent to this activity satellite vents formed along the margin of this structure collapse of the ancestral atka volcano produced a caldera 5 km in diameter and was accompanied by the eruption of a large dacitic flow big pink after caldera formation four major volcanic cones korovin mount kliuchef sarichef and konia grew around the margins of atka caldera this study focuses on the atka volcano korovin and mount kliuchef volcanic centers atka lavas range from basalt through dacite with all major volcanic vents erupting lavas spanning similar compositional ranges they are porphyritic with phenocrysts of plagioclase 25 36 olivine 0 4 3 6 or orthopyroxene 2 8 clinopyroxene 1 12 and a single fe ti oxide 0 2 hydrous phases are lacking even in the most evolved lavas two generations of plagioclase occur in the korovin suite but not in lavas from mount kliuchef korovin plagioclase also displays resorption surfaces and inclusion zones which are either absent in the phenocrysts of mount kliuchef lavas or very rare similarly clinopyroxene in korovin has reaction rims and abundant glass inclusions which are absent in the mount kliuchef lavas lavas erupted in the atka volcanic field vary from 47 to 66 sio2 although the complex is dominated volumetrically by basaltic lavas on major element harker diagrams mafic and intermediate atka volcano lavas form linear trends but the most siliceous lavas do not fall along these trends by contrast mount kliuchef lavas are compositionally bimodal whereas korovin samples range from 51 to 62 wt silica with intermediate lavas clustering around 55 sio2 incompatible trace element abundances of korovin lavas do not increase with silica content but those of atka volcano and mount kliuchef do on incompatible incompatible element diagrams the three suites behave very differently differences also exist between the ree characteristics of the three atka suites the petrographic and geochemical differences between the three suites suggest formation by markedly different crustal magmatic processes the characteristics of the atka volcano suite suggest formation of the basalt to dacite suite by crystal fractionation of an anhydrous phenocryst assemblage incompatible incompatible element trends suggest the mafic mount kliuchef lavas were also produced by crystal fractionation a conclusion supported by quantitative mass balance calculations the difference in slope between the atka volcano and mount kliuchef suites indicates however that there were significant differences in these fractionation schemes modeling as well as geochemical characteristics suggest that the mount kliuchef dacites are not the product of crystal fractionation they may represent 1 crustal melts 2 complete melts of siliceous lava produced during earlier magmatic stages or 3 residual liquids from the atka volcano magmatic system in contrast to these suites the korovin lavas could not have been formed by crystal fractionation rather the andesites are probably the result of mixing basaltic and dacitic magmas the atka data suggest 1 large volcanic centers may be supplied by a series of non communicating crustal magma chambers 2 local factors may be important in determining liquid lines of descent and 3 very different magmatic processes may occur in adjacent magma chambers if the complexity of the atka suite is a common feature of arc magmatic plumbing systems reconstructing differentiation processes requires analytical sample sets which are well constrained spatially and temporally

magmatic metasomatism in peridotites from the zambales ophiolite evidence for variable amounts of melt induced metasomatism in upper mantle peridotites has been observed in the ultramafic section of the zambales range ophiolite luzon republic of the philippines these peridotites are locally cut by mafic dikes that in some places visibly ooze into the host peridotite the mafic silicate magma invaded the peridotite along grain boundaries and crystallized as poikilitic plagioclase and clinopyroxene compositional data indicate that the mafic material reacted with the peridotite mineral compositions in the mafic dikes are distinct from those in the surrounding peridotite the compositions of residual olivine and orthopyroxene in the peridotite change with the amount of mafic magma within the peridotite fe mg exchange predominates the residual olivine and orthopyroxene in the peridotite are enriched in fe large relative enrichments in other magmaphilic elements na ti at are also evident mafic liquid in transit through the upper mantle may act as a metasomatic agent locally enriching the peridotite through which it passes and producing small scale compositional differences in rocks that otherwise have remarkably uniform compositions

assimilation of crustal material by basaltic magma strontium isotopic and trace element data from the edgecumbe volcanic field se alaska the edgecumbe volcanics which range from basalt through rhyodacite have sr contents between 125 and 370 p p m rb contents of 1 to 70 p p m ba ranging from 50 550 p p m and initial l7sr sr ratios between 0 70291 and 0 70404 no simple correlation exists between these components and silica the highest sr sr values occur in a group of intermediate lavas 55 60 wt per cent sio2 while the rhyodacites have initial ratios between 0 7035 and 0 7038 with increasing silica sr increases to a maximum in the andesites and then steadily decreases ba and rb increase over the same compositional range the highest l7sr 86sr lavas have major and trace element concentrations which depart from trends defined by most of the lavas the variation in strontium isotopic compositions suggests interaction between parental basaltic magma and crustal material attempts to model the assimilation process using fixed end member assimilation and assimilation fractional crystallization models have failed to produce the observed chemical trends because the parental basaltic liquid underwent little fractionation the variability in hybrid lavas is attributed to variation in contaminant composition initial melts were low in cao al2oj mgo and sr and enriched in sio2 k2o na2o rb and ba as melting progressed melts became enriched in the more refractory components because hybrid strontium isotopic composition is a function of sr concentration as well as isotopic compositions the sr content of the assimilant strongly influences resultant isotopic systematics the development of the assimilants suggests plagioclase was a residual phase during early melting this model of crustal assimilation represents one end member in the spectrum of processes responsible for the generation of continental volcanic suites

a test of a quartz eclogite source for parental aleutian magmas a mass balance approach the observation that magmatic conduits evolve with time has provided a new framework within which to interpret aleutian compositional and isotopic data accordingly a two stage physical model of aleutian magma genesis has been developed and used to model quantitatively geochemical and isotopic mass balance relations parental magmas approximated by high alumina basalts are presumed to be generated by partial melting of eclogite and subducted sediment the incorporation of sediment produces elevated parental magma sr and pb isotopic ratios but lowered epsilon nd relative to morb if these magmas ascend along well established conduits their isotopic characteristics are little changed during ascent in immature conduits interaction with oceanic lithosphere produces significant isotopic variability according to this model the isotopic and geochemical characteristics of parental magmas are fixed by mixing eclogite and sediment melts as a test we have quantitatively examined the mass balance rel

scaling of earthquake rupture growth in the parkfield area self similar growth and suppression by the finite seismogenic layer 1 we propose a new framework on the scaling of earthquake rupture growth time history and we scale the moment rate and the cumulative moment functions of earthquakes over a wide magnitude range mw 1 7 6 0 in parkfield california the moment rate and the cumulative moment functions of the small and medium earthquakes mw 1 7 4 6 are derived by slip inversion analyses with the empirical green s function technique the moment rate functions of the investigated earthquakes except the mw 6 0 event are similar to each other increasing rapidly in the first half growth stage and decelerating in the latter half decline stage in the growth stage the cumulative moment functions are approximated by mo t nm 2 1017 t s 3 independent of the final size of the earthquakes the proportionality of the cumulative moment to the cube of time implies self similarity during earthquake rupture growth in the decline stage the cumulative moment function veers off the common rupture curve the mw 6 0 event also grows along the same rupture curve until 1 s after which the cumulative moment function is proportional to time from the onset itself this is because the finite seismogenic layer limits the vertical extent of dynamic rupture our method and results contribute to our understanding of earthquake source physics especially on earthquake rupture growth processes which may help to improve earthquake early warning techniques

large scale potential vorticity anomalies and deep convection the effect on deep convection of large scale potential vorticity pv anomalies and their associated tropospheric stable layers is complex and not well understood this article examines the meteorological events of 9 july 2007 iop7b of the convective and orographically induced precipitation study cops which was dominated by an upper level pv anomaly that stretched from the uk to southern france and as far northeast as denmark three precipitation regions were identified from the case lines of intense storms beneath the pv anomaly less intense more widespread convective precipitation to the east of the pv anomaly and in between a region of no precipitation the latter of these coincided with the high resolution measurements and model analyses from cops the extensive and varied data analyzed in this investigation show that convective available potential energy cape was present in this region the distribution of cape and convective inhibition cin is presented via an innovative pseudo 3d visualization that allows horizontal and vertical interactions to be considered however convection was capped by a complex arrangement of dry layers the base of the key layer was at 750 hpa these dry layers descended separately from the upper troposphere moving around the pv anomaly as it developed from a breaking rossby wave to the west during the seven days before the iop this case adds to other studies that show that descent of complex dry layers is an important mechanism for forming convection inhibiting atmospheric lids in western europe a simple conceptual model is developed that synthesizes the effect of large scale pv anomalies on deep convection from a series of consistent case studies this model has significant implications for storm forecasting and projections of storminess in future climates as it highlights the importance of thin structures that can advect hundreds of km before having an impact copyright 2012 royal meteorological society

rupture characteristics of the three m 4 7 1992 1994 parkfield earthquakes slip on the san andreas fault was determined for three m 4 7 earthquakes using a tomographic inverse system beroza and spudich 1988 to invert seismic source time functions stfs from s waves stfs were obtained by deconvolving mainshock accelerograms by those from collocated smaller earthquakes accelerograms were from the u s geological survey parkfield small aperture array upsar and from a distributed array of digital accelerometer stations at parkfield eight or nine stfs are used in each of the three inversions stfs are typically symmetrical pulses with a duration of about 0 3 0 5 s in the inversion mainshock rise time was set to 0 05 s and we allowed the rupture time to vary slightly from a constant rupture velocity of approximately 0 85 rupture for all three events which are located in or close to the middle mountain preparation zone or box mmb quickly reaches a local maximum in slip and then propagates outward to peaks ridges or plateaus in the slip distribution slip for the october 20 1992 event located just inside the southern edge of the mmb propagates from an initial spike north and updip along a curving ridge for about 2 km the initial spike continued to grow in the november 14 1993 event located north of the october 20 1992 event just beneath the hypocenter of the 1966 parkfield earthquake which shows little directivity although there is a smaller patch of slip updip and to the south in contrast rupture for the december 20 1994 event located just south of the october 20 1992 event propagated north and slightly updip creating a rough plateau in slip a few kilometers wide on a side directivity for this event also is to the north directivity for all three events points in the approximate direction of the 1966 hypocenter small pulses which comprise a coda are found on the stfs for several seconds after the initial impulsive event several tests based on the assumption that the average of all stfs from upsar for each event is an estimate of the true slip at the source suggest that the codas in the stfs are s waves from a long duration source rather than uncorrected site response an initiation phase is found on the array average for the november 14 1993 and december 20 1994 events these precursory phases are the result of a spike in slip at the hypocenter a value of 2 4 4 mm is obtained for d c the slip weakening distance by interpreting the initial spike as a critical patch the few aftershocks for the october 20 1992 event are distributed to the north and updip of the mainshock but the november 14 1993 event had a strong burst of aftershock activity that propagated to the north of its hypocenter at roughly the same depth aftershocks of the december 20 1994 event are mostly updip the november 14 1993 event had the simplest slip distribution appeared to be the most impulsive and had the most active aftershock sequence and the greatest depth if the eventual parkfield earthquake initiates near the 1966 hypocenter then the directivity of the three events studied here will have pointed to it however it is certainly possible that both the initiation of characteristic parkfield shocks and the directivity of smaller events are controlled by fault properties on a larger scale such as by fault bends or jogs

the isotopic and chemical evolution of mount st helens isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the mt st helens volcano suggest three different episodes 1 40 000 2500 years ago eruptions of dacite with end 5 esr 10 variable 18o 206pb 204pb 18 76 ca sr 60 rb ba 0 1 la yb 18 2 2500 1000 years ago eruptions of basalt andesite and dacite with end 4 to 8 esr 7 to 22 variable 18o thought to represent melting of differing mantle crust reservoirs 206pb 204pb 18 81 18 87 variable ca sr rb ba la yb and high zr 3 1000 years ago to present day eruptions of andesite and dacite with end 6 esr 13 18o 6 variable206pb 204pb ca sr 77 rb ba 0 1 la yb 11 none of the products exhibit eu anomalies and all are lree enriched there is a strong correlation between87sr 86sr and differentiation indices these data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet but not feldspar followed by intrusion of this crustal reservoir by mantle derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re equilibration of pb isotopic compositions than other components suggesting that pb is being transported via a fluid phase the nd and sr isotopic compositions lie along the mantle array and suggest that the mantle underneath mt st helens is not as depleted as morb sources there is no indication of seawater involvement in the source region

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

boundary layer moisture regimes data from fifty two aircraft flight legs 100 150m above ground from hapex and fife are analysed to estimate characteristics of boundary layer moisture fluctuations in boundary layers with different bulk stability and surface energy regimes in hapex considerable effort was devoted to the quality of measurement of moisture fluctuations the data include repeated 120km flight legs flown at 150m over the same relatively homogeneous terrain allowing statistical examination of motion characteristics on horizontal scales up to 10km this study finds two prototype boundary layer regimes with significant boundary layer instability relatively weak surface evaporation and drier air aloft the entrainment drying boundary layer may develop this boundary layer is characterized by vertical divergence of the moisture flux and significant diffusion of dry air from the top downwards top down in this type of boundary layer dry air occasionally reaches the lower boundary layer leading to negative moisture skewness in spite of positive temperature and vertical velocity skewness associated with warm moist updraughts in contrast the moistening boundary layer associated with greater surface evaporation is characterized by positive moisture skewness near the surface in addition to the above turbulent moisture fluctuations some of the above data are characterized by 10km moisture variations with horizontal gradients often concentrated in narrow zones of horizontal convergence since corresponding signatures of vertical velocity and temperature are weaker these zones are referred to as mesoscale moisture fronts as a more general feature moisture variations on scales of 5km and greater are negatively correlated with temperature variations associated with cool moist regions and warm dry regions on scales of tens of kilometres and greater such negative correlation may be due to inhomogeneity of the surface energy budget the negative moisture temperature correlation leads to large mesoscale variations of relative humidity and the lifted condensation level

the orographic impact on patterns of embedded convection during the august 2005 alpine flood convective precipitation structures during an alpine heavy precipitation event in august 2005 are investigated utilising a mesoscale non hydrostatic numerical model and observational data the focus is on the mechanisms of convective initiation during the very beginning of the event when organised cellular updraughts enhanced the precipitation over the southern german alpine foreland a set of sensitivity experiments with systematically modified topography is conducted in order to investigate the role of single topographic obstacles in initiating and arranging convection a reference simulation proves to be capable of qualitatively reproducing the development of two distinct flow parallel aligned non stationary convective precipitation bands which are observed by radar and surface measurements the simulations reveal the presence of two low level convergence lines that provide a primary mechanism for both initiating and aligning convective cells during the analysed period mountains embedded in the alpine foreland are not decisive for triggering and aligning the convective cells in this case since the primary mechanism provides a consistent forcing for the convective precipitation bands however the experiments demonstrate a dependence of the alignment and intensity of one of the precipitation bands upon the position of the pre existing convergence line relative to a flow parallel aligned mountain ridge enhancement weakening of the existing banded structure and convective intensity is achieved if the convergence line is located upstream downstream of the ridge with respect to a cross ridge low level inflow the weakening mode is characterised by a subsiding cross ridge inflow into the convergence line that stabilises the atmosphere and therefore inhibits the formation of a continuous convective precipitation band copyright 2011 royal meteorological society

lead isotopic data from sulfide minerals from the cascade range oregon and washington abstract lead isotopic studies of mineral deposits associated with tertiary plutons found in the cascade range of oregon and washington demonstrate a rather uniform isotopic composition in various sulfide minerals 206 pb 204 pb 18 84 to 19 05 207 pb 204 pb 15 57 to 15 62 208 pb 204 pb 38 49 to 38 74 show less variation than data from the volcanic rocks of the cascade range and fall within the mixing array defined by the morb regression line and continental sediments an evaluation of the role of crustal assimilation by hydrothermal convection during emplacement was made on five sulfide deposits associated with a single composite batholith the cloudy pass pluton the pb isotopic data and mass balance calculations suggest that only minor amounts of the lead were derived from the overlying precambrian swakane biotite gneiss during emplacement the bulk of the metal that occurs in sulfide deposits in the cascade mineral belt appears to have been derived from subducted continental detritus the variation of the pb isotopic signature of sulfides from specific districts or deposits suggests that there is a correlation with age and structure of the crust 206 pb 204 pb is greater than 18 92 in northern washington and southern oregon where deposits have intruded mesozoic or older crust however the ore deposits between the northern oregon border and central oregon south of eugene have intruded younger crust composed largely of mafic and andesitic volcanic rocks and 206 pb 204 pb lies between 18 84 and 18 92 this region previously called the columbia embayment appears to be underlain by tertiary volcanic rocks lead isotopic data may be used to define the boundaries between discontinuous blocks of mesozoic crust and tertiary volcanic cover

limitation of the predominant period estimator for earthquake early warning and the initial rupture of earthquakes abstract we investigate the characteristics and limitations of the parameter p t which represents the predominant period of the p wave we analyze the effects of the length of the time window tw low pass filter and anelastic attenuation as well as the complexity of the source process to elucidate whether values of or the time dependent maximum value of p t have direct relationships to the physical quantities of earthquakes we find that values of have upper and lower limits for larger earthquakes with source durations longer than the tw the values of have an upper limit that depends on the tw on the other hand the values for smaller earthquakes have a lower limit that is proportional to the sampling interval for intermediate earthquakes the values of are close to their typical source durations and can have a large variety due to the complexity of the source process these two limits and the slope for intermediate earthquakes yield an apparent final size dependence of in a wide magnitude range as a result the dependence of on the final size of earthquakes does not suggest that the final size of an earthquake is controlled by processes in the initial part of rupture it is impossible to conclude whether the earthquake is deterministic or not from the dependence of on the final size of earthquakes this is because does not always have a direct relation to the physical quantities of earthquakes

assimilation of peridotite in zoned calc alkaline plutonic complexes evidence from the big jim complex washington cascades the big jim complex is a concentrically zoned ultramafic to felsic plutonic complex which intruded the pelitic chiwaukum schist most of the major plutonic rock types from websterite through hornblendite gabbronorite hornblende gabbro and diorite to granodiorite enclose harzburgite and metaperidotite xenoliths similar to foliated metaperidotite lenses included in the chiwaukum schist the larger xenoliths preserve tectonite fabrics all have mg s mole fraction mgo mgo feo from 0 90 to 0 89 the same as those of chiwaukum metaperidotites and distinctly different from undeformed big jim dunite mg s 0 84 to 0 82 and websterite 0 82 to 0 78 contact relations indicate widespread stepwise replacement of harzburgite by pyroxenite hornblendite gabbro and diorite thermodynamic modelling using an expanded regular solution model for silicate liquids ghiorso 1985 ghiorso and carmichael 1985 predicts that reaction between olivine fo90 and a liquid with the composition of big jim diorite 1 5 wt h2o at 1 100 c and 3 kb would produce websterite mg s 0 75 to 0 81 and dunite 0 79 to 0 82 this process is exothermic and results in a negative change in volume since it increases total solid mass under conditions of decreasing temperature modelled crystal fractionation with assimilation of olivine reproduces important features of the chemical variation observed in the big jim complex where crystal fractionation alone fails the big jim complex has affinities with other ultramafic to felsic plutonic complexes such as the bear mountain complex snoke et al 1981 1982 and the emigrant gap complex james 1971 the latter have wehrlite and clinopyroxenite rather than websterite but both have concentric zoning with olivine bearing rock types surrounded by successively more felsic pyroxenite gabbro and diorite in general concentrically zoned complexes of this type may form where magma reacts with mantle derived wall rock or ultramafic cumulates assimilation of peridotite in fractionating magma may be important in subduction related magmatic arcs

repeating earthquake finite source models strong asperities revealed on the san andreas fault 1 we investigate the rupture process of a sequence of repeating mw 2 1 earthquakes on the san andreas fault in parkfield spanning the occurrence of the september 28 2004 mainshock by inverting seismic moment rate functions obtained from empirical green s function deconvolution the results show that these events have extremely concentrated slip patches with radii on the order of 10 20 m with peak slip between 8 4 and 11 4 cm the rupture speed and rise time are consistent with values of larger earthquakes the spatial distribution of stress drop for the events shows low average values 2 5 5 6 mpa and very large peak values of 66 7 93 9 mpa the results show that strong asperities can exist at small scales on an otherwise weak fault and helps reconcile differences between traditional spectra based and tectonic loading methods for determining the stress drop of small repeating earthquakes

rupture characteristics of the 2005 tarapaca northern chile intermediate depth earthquake evidence for heterogeneous fluid distribution across the subducting oceanic plate 1 we examined the rupture of the 2005 tarapaca northern chile earthquake at about 110 km depth with respect to both kinematic and dynamic characteristics by using regional and teleseismic waveforms the earthquake has a downdip tensional focal mechanism the subhorizontal rupture is characterized by two patches of large slip and high stress drop which are aligned nearly in the east west direction being perpendicular to the direction of the chile trench rupture initiated in the eastern patch and then propagated to the western patch between the two patches there exists a region of nonpositive stress drop and high strength excess which can cause subshear rupture to propagate from the eastern to the western patches but radiates little seismic waves seismic radiation energy from this earthquake tends to be low which is consistent with the nonpositive stress drop and high strength excess between the two patches while the physical mechanism of intermediate depth earthquakes is still controversial current leading hypotheses are associated with dehydration within subducting plates the rupture characteristics of the tarapaca earthquake can be related to heterogeneous fluid distribution due to the dehydration the spatial separation and dominant stress of the two large slip patches agree with the characteristics of the previously reported double seismic zone beneath chile the two patches may be the manifestation of the double seismic zone where dehydration reactions can release fluid using a numerical simulation of 3 d dynamic rupture we have shown that weakening due to fluid can account for the rupture characteristics of the tarapaca earthquake

mid pleistocene basalt from the seguam volcanic center central aleutian arc alaska local lithospheric structures and source variability in the aleutian arc recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern aleutian arc and the sources and processes that produce the broad spectrum of aleutian basaltic lavas mid pleistocene basalt from the seguam island volcanic center in the central aleutian arc erupted through strongly extended arc crust contains 4 8 mgo is low in k2o 18 and is distinguished from aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements ree and low sr and pb content the ba la 47 72 and pb la 0 7 1 5 ratios are high incompatible element ratios are variable and ree patterns cross one another the 87sr 86sr ratio 0 70365 0 00003 is the highest value obtained from the aleutian islands also 206pb 204pb 18 91 0 02 207pb 204pb 15 60 0 02 and 208pb 204pb 38 51 0 04 ratios are among the most radiogenic values known whereas the 143nd 144nd ratio 0 51297 0 00003 is among the lowest measured low abundances of incompatible elements high ba la and pb la ratios uniform sr nd and pb isotope ratios and high eruptive temperatures 1160 c suggest that basalt erupted at seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics trace element abundances rule out an eclogite source and suggest instead a peridotite dominated source with 5 10 of a subducted sediment component this peridotite may reside in the mantle wedge or in the amlia fracture zone afz of the pacific plate that was subducted beneath seguam 1 ma in addition the afz possibly enhanced the subduction of terrigenous sediment into the source region beneath seguam and this extra terrigenous component may explain the ba la pb la and sr nd and pb isotope ratios the distinctive composition of seguam basalt reflects a complex set of physical conditions including fracture zone subduction and intraarc extension that affected both the source region and thermo chemical history of magma ascent local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the aleutian arc

frictional properties on the san andreas fault near parkfield california inferred from models of afterslip following the 2004 earthquake the abundance of geodetic and seismic data recording postseismic deformation following the 2004 parkfield earthquake provides an unprecedented opportunity to resolve frictional properties on the parkfield section of the san andreas fault the parkfield segment is a transition between the locked section to the southeast that last ruptured in the 1857 fort tejon earthquake and the creeping section to the northwest we develop three dimensional rate and state dependent friction models of afterslip following the 2004 earthquake to investigate the frictional behavior of the fault it is assumed that the coseismic rupture occurred on an area of the fault surrounded by aseismic creep that accelerated after the earthquake we estimate the distribution of coseismic slip afterslip and rate state frictional parameters by inverting a two step slip model in the model we 1 estimate the coseismic slip distribution from 1 hz global positioning system gps data and 2 use the corresponding coseismic shear stress change on the fault as input into a numerical afterslip model governed by rate state friction we find the rate state frictional parameter a b an indicator of frictional stability is in the range 10 4 10 3 at 50 mpa normal stress which is near the transition from potentially unstable negative a b to nominally stable positive a b friction the estimate of a b values falls within a wide range of experimental values reported for serpentinite which crops out along the san andreas fault zone the critical slip distance d c which characterizes the distance over which strength breaks down during a slip event is in the range 0 01 0 1 m consistent with seismic estimates and a fault gouge thickness of 1 10 m the afterslip model reproduces most features observed in the gps time series data including high surface velocities in the first few months after the earthquake and lower rates at later times as well as the cumulative postseismic displacement the model tends to underpredict the displacement data at later times suggesting that perhaps the modeled afterslip period ends too quickly or an unmodeled deformation process dominates the signal at later times

lead isotopic compositions of south sandwich island volcanic rocks and their bearing on magmagenesis in intra oceanic island arcs pb isotope ratios have been measured in 12 volcanic rocks from the south sandwich islands the ranges are 206pb204pb 18 51 18 66 207pb204pb 15 55 15 64 208pb204pb 38 42 38 64 in 207pb204pb 206pb204pb and 208pb204pb 206pb204pb correlation diagrams the south sandwich data plot distinctly above the fields for ocean ridge basalts and yield trends showing apparent mixing with a sedimentary end member similar to south atlantic pelagic sediments as reported by chow and patterson 1962 and this study armstrong and cooper 1971 have likewise shown that volcanics from the lesser antilles show mixing trends with north atlantic sediments in pb isotope correlation diagrams the north atlantic sediments have distinctly higher 206pb204pb and 208pb204pb ratios compared to the south atlantic sediments the parallel relationships between sediments and volcanic island arc rocks of the north and south atlantic provide strong evidence for a component of pb from subducted sediments in the lavas of the west atlantic basin in contrast to these data lavas from the mariana arc in the western pacific show little or no component of pb from pelagic sediments the reason for the different behaviors in the two settings is speculative

aleutian lead isotopic data additional evidence for the evolution of lithospheric plumbing systems abstract lead isotopic ratios and concentrations have been measured in lavas from the aleutian volcanic centers of adak 12 and atka 12 whereas lavas from atka have very small pb isotopic ranges 206 pb 204 pb 18 78 18 86 207 pb 204 pb 15 55 15 62 208 pb 204 pb 38 31 38 52 those from adak have ranges nearly twice as large 206 pb 204 pb 18 34 18 86 207 pb 204 pb 15 47 15 63 208 pb 204 pb 37 81 38 56 on lead lead isotopic diagrams atka data form tight clusters whereas those from adaak define a well developed linear trend extending from the atka field toward less radiogenic values at a given 206 pb 204 pb the lavas from both islands plot in the morb trend for 208 pb 204 pb but atka lavas plot above the morb 207 pb 204 pb 206pb 204pb trend and those from adak fall both within and above the trend lead contents in lavas from atka increase four fold 5 21 ppm over the compositional range of the volcanic suite 49 68 sio2 in contrast actak lavas 49 65 have concentration levels of 0 6 13 ppm and display no simple correlation with sio2 the lead isotopic data alone can be explained by three different processes model 1 assigns lead isotopic differences to original magma source heterogeneity according to model 2 the isotopic ratios of a primary non radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab derived component in contrast model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non radiogenic lithospheric component during conduit formation because these models all adequately explain the lead isotopic data supplementary geologic petrographic geochemical and isotopic data must be used to select the most likely model careful consideration of the evidence suggests model 3 best explains our extensive lead isotopic data as well as other characteristics of aleutian lavas our study suggests detailed isotopic studies of individual volcanic centers arc or otherwise can be extremely useful in understanding the complex processes of magma generation extraction ascent and evolution

possible petrogenetic relations between low and high mgo aleutian basalts although the volumetrically dominant lava in the aleutian arc is basaltic its petrographic geochemical and isotopic character is variable and the petrologic significances of the different basalts have been much debated to evaluate possible petrogenetic relations all available petrographic major trace and rare earth element and isotopic data from aleutian basalts n 205 have been collected on the basis of mgo content basaltic 2 lavas have been divided into three major classes high magnesia basalts transitional basalts and low mgo basalts low mgo basalts have less than 6 mgo and constitute 67 of the analyzed basalts using al 2 o 3 content this basalt class has been further divided into two subclasses low alumina basalts the less abundant class 15 of the low mgo basalts have less than 18 wt al 2 o 3 and 450 ppm sr and high tio 2 1 0 wt feo t 9 and incompatible element abundances high alumina basalts 18 wt al 2 o 3 are characterized by low tio 2 1 wt and 9 feo t low incompatible and compatible element contents and high sr abundances 450 ppm transitional basalts are defined as basalts with between 6 and 9 wt mgo high magnesia basalts mgo 9 wt constitute 10 of the analyzed aleutian basaltic lavas and occur at only a few volcanic centers fractionation of high magnesia basaltic magma is unlikely to produce magma compositions similar to high alumina basalts the compositional data are however consistent with assimilation of lithospheric material by high alumina basaltic liquid to produce high magnesia basaltic magmas and generation of low alumina basalts by crystal fractionation of these hybrid liquids within this context high alumina basaltic magmas are considered primary

dynamic high speed rupture from the onset of the 2004 parkfield california earthquake 1 we investigated the 2004 parkfield earthquake using a multiscale slip inversion the multiscale approach allows us to both focus on the details of the early stages of the rupture process and to model the entire earthquake at a larger scale our model indicates that the parkfield earthquake began with bilateral high speed rupture with fast slip rate up to 4 m s and fast propagation velocity of about 3 0 km s rupture propagation to the southeast was arrested 3 s after onset while propagation to the northwest continued generated a large slip patch at 5 s and ceased within 10 s a similar rapid initiation was observed for the 2004 mid niigata prefecture japan earthquake in a previous study high speed initial rupture suggests that large earthquakes may initiate and grow in the same way as small earthquakes and that the final size is difficult to predict from the early stages of an earthquake

isotope and trace element geochemistry of augustine volcano alaska implications for magmatic evolution k e johnson r s harmon j m richardson s moorbath and d f strong department of geology 1112 turlington hall university of florida gainesville fl 32611 usa us army research office p o box 12211 research triangle park nc 27709 usa geoscience branch ontario geological survey willet green miller centre sudbury ont pse 6b5 canada department of earth sciences university of oxford parks road oxford oxi 3pr uk centre for earth and ocean research university of victoria victoria b c v8w 3p6 canada

properties of the seismic nucleation phase abstract near source observations show that earthquakes begin abruptly at the p wave arrival but that this beginning is weak with a low moment rate relative to the rest of the main shock we term this initial phase of low moment rate the seismic nucleation phase we have observed the seismic nucleation phase for a set of 48 earthquakes ranging in magnitude from 1 1 8 1 the size and duration of the seismic nucleation phase scale with the total seismic moment of the earthquake suggesting that the process responsible for the seismic nucleation phase carries information about the eventual size of the earthquake the seismic nucleation phase is characteristically followed by quadratic growth in the moment rate consistent with self similar rupture at constant stress drop in this paper we quantify the properties of the seismic nucleation phase and offer several possible explanations for it

loading mechanism and scaling relations of large interplate earthquakes we have done a numerical simulation of tectonic loading at transform plate boundaries with a lithosphere asthenosphere coupling model subject to steady relative plate motion in this model a seismic fault is represented by a locked patch with a finite length on an infinitely long vertical dislocation surface cutting the entire lithosphere through the numerical simulation we have obtained the following results stress accumulation on the fault is partly due to viscous drag at the base of the lithosphere base loading and partly due to dislocation pile ups at horizontal edges of the fault edge loading if the viscosity of the asthenosphere is less than 1018 pa s the base loading is not effective since its rate exponentially decays very soon if the viscosity of the asthenosphere is greater than 1020 pa s both types of loading are effective and then the stress accumulation rate in the early stage of loading is formally written as d dt vpl a bl where vpl is a rate of relative plate motion and l is a fault length when the fault length l is small the effect of edge loading the second term is dominant and the stress accumulation rate is in proportion to the inverse of l when the fault length l is large the effect of base loading the first term becomes dominant and the stress accumulation rate is independent of l it is well known that the general l cubed dependence of seismic moment m0 breaks for large interplate earthquakes this break in the moment length relation can be ascribed to difference in loading mechanism between small and large earthquakes from the results of numerical computation if interplate earthquakes have the same stress drop regardless of their fault lengths we may derive a l squared dependence of m0 for large interplate earthquakes and a linear l dependence of m0 for very large interplate earthquakes

on moment length scaling of large strike slip earthquakes and the strength of faults 1 several large strike slip earthquakes have occurred in various tectonic settings in the past 5 years adding well documented data to the global collection of moment and length estimates for such earthquakes based on this augmented dataset we reexamine the controversial issue of scaling of seismic moment with length of rupture we find that the global dataset of large strike slip earthquakes follows a bi modal distribution most oceanic and or intraplate strike slip earthquakes have stress drops that are 5 times larger than interplate continental ones when distinguishing these two classes the scaling is compatible with that predicted by simple dislocation theory

spatial and temporal stress drop variations in small earthquakes near parkfield california 1 we estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the parkfield segment of the san andreas fault in central california we use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed p wave source spectra with a spatially varying empirical green s function egf our brune type stress drop estimates vary from 0 1 to over 100 mpa with a median value of 6 75 mpa which is nearly constant with moment implying self similarity over the ml 0 5 to 3 0 range of our data the corner frequency decreases for earthquakes at shallower depths consistent with slower rupture velocities and reduced shear wave velocities in local velocity models the estimated median stress drops show significant lateral variations we find lower stress drops in the middle mountain asperity and along the creeping fault section and higher stress drops in the hypocentral region of the 2004 m6 0 parkfield earthquake the main shock did not alter the overall pattern of high and low stress drop regions however a statistical test reveals areas with significant changes in computed stress drops after the main shock which we compare to estimated absolute shear stress changes from a main shock slip model by calculating t from the spectral egf ratio we also identify areas with increased attenuation after the main shock and we are able to distinguish source effects and near source attenuation effects in the spectral analysis these results are confirmed independently from spectral ratios of repeating microearthquake clusters

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

pb and sr isotopic systematics of some basalts and sulfides from the east pacific rise at 21 n project rita abstract tholeiitic basalts and sulfide deposits from the cyana and alvin diving programs rita project on the east pacific rise were analyzed for pb and sr isotopes the basalt data plot within the field defined previously by other east pacific rise basalts 206 pb 204 pb 18 35 18 58 207 pb 204 pb 15 48 15 53 208 pb 204 pb 37 8 38 1 87 sr 86 sr 0 7022 0 7025 pb u and sr contents 0 5 0 05 and 110 ppm respectively and values 6 are typical of morb whereas th u ratios 3 5 are significantly higher the pb isotopic ratios of the sulfide samples are very homogeneous 206 pb 204 pb 18 47 207 pb 204 pb 15 49 208 pb 204 pb 37 90 and plot in the middle of the basalt field this indicates that 1 the sulfide pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments and 2 the solutions from which the sulfides were deposited had uniform pb isotopic composition the pb contents of three sulfide samples is relatively high 170 1310 ppm the sr contents of five sulfide samples are widely scattered from 12 to 210 ppm with 87 sr 86 sr ratios intermediate between basaltic and seawater values 0 70554 0 00005 to0 70795 0 00011 leaching experiments show that both basalt derived sr and seawater sr were present in the solutions which deposited the sulfides in some cases sr was also adsorbed from seawater onto the sulfides following their deposition basalt derived sr and seawater sr are also present in associated non sulfide phases

strontium isotopic and selected trace element variations between two aleutian volcanic centers adak and atka implications for the development of arc volcanic plumbing systems major and trace element concentrations and initial 87sr 86sr ratios of lavas from the aleutian volcanic centers of adak and atka have been used to study the evolution of their respective lithospheric plumbing systems the centers are within 150 km of one another and show similar overall silica ranges 47 67 but adak 40 km3 is smaller than atka 200 km3 adak s lavas are chemically and isotopically heterogeneous 87sr 86sr 0 70285 0 70330 and two units contain lithospheric xenoliths the lavas of the much larger atka on the other hand have much less variability in major and trace elements as well as 87sr 86sr 0 70320 0 70345 we suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers because aleutian volcanic centers often remain fixed for relatively long periods of time 5 m y once established magmatic passageways are repeatedly used young plumbing systems are relatively cool and contain large amounts of wallrock contaminant and ascending magmas undergo contamination as well as concurrent crystallization and fractionation with time however heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas in effect the plumbing system matures the chemical heterogeneity displayed by young dirty systems like adak reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation thus it is the petrologic data of mature clean systems like atka that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution

secondary initiation of multiple bands of cumulonimbus over southern britain i an observational case study summary special observing facilities have been assembled in southern england as part of the convective storm initiation project csip to study the mesoscale and convective scale processes that determine precisely where warm season convective showers will break out this paper reports the results of a case study during the pilot field campaign of csip in july 2004 one purpose of the pilot project was to demonstrate the value of various observational facilities and to evaluate the usefulness of a variety of analysis and synthesis techniques amongst other things the case study demonstrates the utility of high resolution imagery from the meteosat second generation satellite for tracking the early stages of the convective clouds and of a new clear air scanning radar at chilbolton for mapping both the top of the boundary layer and the initialgrowth of the convective cells that penetrate it the particular event studied involved the triggering of convection that developed into three parallel arcs of showers and thunderstorms the first arc was triggered along the leading edge of the outflow density current from an earlier cluster of showers but the convection in the second and third arcs was triggered by a different mechanism the paper describes in detail the way in which this convection broke through the stable layer or lid at the top of a boundary layer of variable depth the strength of the lid decreased and the depth of the boundary layer increased with timeas a result of diurnal heating but the precise locations where convection finally broke through were determined by the spatial variability in boundary layer depth the analysis suggests that a wave like modulation of the boundary layer depth of amplitude 150 m perhaps due to a gravity wave disturbance from the earlier cluster of showers had a greater influence on where the convection was triggered than the modest hills typically 200 m high in southern england

development of multiscale slip inversion method and its application to the 2004 mid niigata prefecture earthquake 1 we present a multiscale slip inversion method for earthquakes as a first attempt to analyze an initial rupture process on a small scale and a whole one on a large scale a multiscale source model is constructed by renormalizing slip rate distributions on different scales a multiscale observation equation includes a renormalized kernel matrix using the new method and employing theoretical and empirical green s functions with two small events mw 2 3 and 3 3 we analyze the 2004 mid niigata prefecture earthquake mw 6 6 following a preliminary deconvolution analysis suggesting a complexity of the initial phase the multiscale analysis reveals details of the initial stage of the rupture process the first 1 s successfully in the multiscale model the estimated source process is consistent for all scales while independent slip inversion analyses on three scales monoscale analysis result in inconsistent slip distributions with large errors the maximum slip rate is about 1 0 m s and the rupture velocity is 2 5 3 0 km s in the initial rupture process four stages of rupture growth with different rupture directivities are found the first 0 4 s with northeastward directivity from 0 6 s to 1 0 s with southward directivity until 2 0 s with northeastward directivity again and after 2 0 s with southwestward directivity these stages may represent cascading ruptures evolving into a large earthquake the image of the whole rupture process implies self similarity of the dynamic rupture process and is a breakthrough in the complete realization of earthquake source scaling

slip on the san andreas fault at parkfield california over two earthquake cycles and the implications for seismic hazard parkfield california which experienced m 6 0 earthquakes in 1934 1966 and 2004 is one of the few locales for which geodetic observations span multiple earthquake cycles we undertake a comprehensive study of deformation over the most recent earthquake cycle and explore the results in the context of geodetic data collected prior to the 1966 event through joint inversion of the variety of parkfield geodetic measurements trilateration two color laser and global positioning system including previously unpublished two color data we estimate the spatial distribution of slip and slip rate along the san andreas using a fault geometry based on precisely relocated seismicity although the three most recent parkfield earthquakes appear complementary in their along strike distributions of slip they do not produce uniform strain release along strike over multiple seismic cycles since the 1934 earthquake more than 1 m of slip deficit has accumulated on portions of the fault that slipped in the 1966 and 2004 earthquakes and an average of 2 m of slip deficit exists on the 33 km of the fault southeast of gold hill to be released in a future perhaps larger earthquake it appears that the fault is capable of partially releasing stored strain in moderate earthquakes maintaining a disequilibrium through multiple earthquake cycles this complicates the application of simple earthquake recurrence models that assume only the strain accumulated since the most recent event is relevant to the size or timing of an upcoming earthquake our findings further emphasize that accumulated slip deficit is not sufficient for earthquake nucleation online material model fault geometry fit to the data for the inversions and model resolution

petrology and geochemistry of the island of sarigan in the mariana arc calc alkaline volcanism in an oceanic setting isotopic studies of rocks from oceanic island arcs such as the marianas indicate that little if any recycling of continental material e g oceanic sediments occurs in these arcs because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs an important question is whether the andesites of continental arcs are produced by a fundamentally different more complex mechanism than the lavas of oceanic arcs an excellent opportunity for study of this question is provided by the island of sarigan in the mariana active arc on which calc alkaline andesites including hornblende bearing types are exposed along with more mafic lavas

phase relations of a high mg basalt from the aleutian island arc implications for primary island arc basalts and high al basalts many volcanic centers in the aleutian islands have erupted lavas that range in composition from high mg basalt mgo 9 wt to more fractionated and voluminous high al basalts and basaltic andesites the petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated the phase relations of a typical high mg basalt from the makushin volcanic field on unalaska island provide important constraints on petrogenetic models results of one atmosphere and moderate to high pressure 5 20 kb anhydrous experiments are similar to results obtained from primitive morb at low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize glasses in equilibrium with olivine and clinopyroxene at intermediate pressure 5 to 10 kb are similar in composition to high al basalt plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly although our studies do not confirm the primary nature of high mg basalts they do support a model in which high al basalts are generated by moderate amounts of crystal fractionation from more primitive high mg mg fe lower al2o3 basaltic magmas near the arc crust mantle boundary

observations of the development of convection through a series of stable layers during the convective storm initiation project this article describes the development of convection through a series of stable layers on 20 july 2004 during the pilot project of the convective storm initiation project csip convective clouds developed ill south central england as a result of diurnal heating they formed near the south coast and grew gradually as they moved northwards eventually producing light precipitation several stable layers and associated dry layers or lids were observed by sequential radiosondes launched every two hours most of these lids were also detected by the chilbolton l band radar and with a wind profiler as prominent layers of bragg echo one particular lid at 820 mb 1 7 km inhibited the early development of the convection for about an hour delaying the production of precipitation the uppermost lid at 650 mb stopped all convection from intensifying in the csip domain estimates of the growth of convection through the lids were sensitive to the values of temperature and humidity chosen to represent the ascending parcels the development of the thermals in the boundary layer was best explained by using the surface layer average values parcels would not have been sufficiently buoyant to reach the lifting condensation level lcl unless these values were used however such parcels in the absence of mixing would then have been too buoyant to be capped by the main lid at 1 7 km as was observed by the radars in fact the development of the cumulus clouds above the lcl is best represented by using 950 1000 mb average thermodynamic values the results highlight the importance of predicting the altitude strength and persistence of such lids in order to forecast the intensity location and timing of convective precipitation more accurately copyright c 2008 royal meteorological society

geodetic data inversion using a bayesian information criterion for spatial distribution of fault slip summary we developed a new inversion method to reconstruct static images of seismic sources from geodetic data using akaike s bayesian information criterion abic coseismic surface displacements are generally related with a slip distribution on a fault surface by linear integral equations parametric expansion of the fault slip distribution by a finite number of known basis functions yields a set of observation equations expressed in a simple vector form incorporating prior constraints on the smoothness of slip distribution with the observation equations we construct a bayesian model with unknown hyperparameters the optimal values of the hyperparameters which control the structure of the bayesian model are objectively determined from observed data by using abic once the values of hyperparameters are determined we can use the maximum likelihood method to find the optimal distribution of fault slip we examined the validity of this method through a numerical experiment using theoretical data with random noise we analysed geodetic data associated with the 1946 nankaido earthquake ms 8 2 by using this method the result shows that the fault slip distribution of this earthquake has two main peaks of 4 and 6 m located off kii peninsula and muroto promontory these two high slip areas are clearly separated by a low slip zone extending along kii strait such a slip distribution corresponds with the fact that the rupture process of this earthquake in the western part is notably different from that in the eastern part

high amplitude seismic waves radiated from the bright spot of an earthquake abstract in the source region of a large shallow earthquake there is likely to be a small spot characterized by the production of high accelerations and a gap in the aftershocks on the seismograms of most large shallow earthquakes and on some seismograms of moderate main shocks two types of p waves may be detected the relatively low amplitude waves following phase p 1 and the predominant high frequency waves following phase p 2 the duration of the seismic waves following phase p 1 the p 1 p 2 time increases proportionately with the earthquake magnitude this fact strongly suggests that the preliminary rupture period is closely related to the radiation of the predominant high frequency seismic waves to explain the above observation the shear strain waves radiated from a moving rupture are calculated because the seismic wave velocity is generally higher than the rupture propagation velocity maximum shear strains by seismic waves usually precede the rupture front if the total shear strain which is equal to the static pre earthquake shear strain plus dynamic shear strain excited by seismic waves exceeds the critical shear strain 10 4 the secondary ruptures break out in the forward direction from the initial rupture the rupture processes of large or moderate earthquakes occur in two stages one is the initiation of rupture corresponding to phase p 1 and the other is the development of secondary ruptures triggered by seismic shear strain waves the latter process generates an earthquake bright spot which radiates high amplitude seismic waves

symmetrical and segmented variation of physical and geochemical characteristics of the central american volcanic front abstract the regional variation of physical and geochemical characteristics of central american volcanoes occurs in two fundamentally different patterns the first pattern is symmetrical about nicaragua crustal thickness silica contents of mafic lavas and volcanic edifice heights are lowest in nicaragua and increase smoothly toward costa rica to the south and guatemala to the north magma density is maximum in nicaragua and decreases smoothly outward the regional variation in crustal thickness is just enough so that magma densities calculated at appropriate moho pressures are the same at the base of the crust throughout the region this is consistent with magma ponding at the base of the crust the bulk compositions of central american basalts show the same symmetrical variation suites of nicaraguan basalts plotted in pseudo ternary cmas projections indicate large olivine and plagioclase primary phase volumes toward costa rica and guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller which is consistent with fractionation at increasing depth the second pattern is the segmentation of the volcanic front and the plate margin in general the segmentation strongly affects the spacing and size of volcanic centers at segment boundaries volcanic centers are generally small and unusually widely spaced toward segment interiors volcano spacing and size increase systematically the lil element contents of lavas strongly reflect this pattern for lavas with similar silica contents the larger the volcano the higher the lil element contents the relationships between segmentation volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab the relationship between volcano volume and lil element content is qualitatively in agreement with an open system fractionation model

on the origin of high alumina arc basalt and the mechanics of melt extraction most models of high alumina arc basalt petrogenesis rely heavily on the supposition that the abundances of certain trace elements in particular the relatively unfractionated rare earth element ree patterns and the unusually high concentrations of k rb sr and ba are incompatible with a garnet bearing subducted oceanic crustal quartz eclogite source rock we have carefully examined this apparently unequivocal evidence in light of recent progress on the physics of melt extraction and the heat transfer and mechanics of magma ascent the weakest element of all trace element models involving a quartz eclogite source is the assumption that the element concentrations are fixed at the source and only later modified in the near surface environment we expand on such models by monitoring the concentrations of r ee and major and trace elements during initial melting ascent and extraction of magma this is done by combining calculated cooling curves for ascending magmatic bodies with high pressure phase equilibria the amount that each phase contributes to the melt is monitored along with the composition of the melt and residual solids with quartz eclogite initial melting initiates gravitational instability of the entire source material melt plus solids before melt extraction can occur during ascent of this mush melting increases until the solids can be repacked to free the melt this extraction takes place some 15 20 km above the slab after about 50 per cent melting at which point the melt has a pattern of ree and other trace element concentrations almost identical to those observed in high alumina arc basalts assuming an initial composition equivalent to altered oceanic crust plus 5 per cent pelagic sediment sr abundances are the only ones which are not well matched by this process the major element concentrations of the extracted melt also closely match those of high alumina arc basalt a similar but less detailed evaluation of both fertile and depleted peridotite source rocks yields good agreement for the ree and other trace element concentrations assuming a lree enriched source rock strongly enriched in k rb sr and ba ni cr and co abundances are satisfied only through substantial low pressure fractionation of mafic phases in particular olivine though not rigorously tested such a process may be compatible with the observed major element concentrations of high alumina basalt however the experimentally verified fact that high alumina basalts could never have been in equilibrium with either an olivine beari ng magma or source rock eliminates this possibility altogether thus the simultaneous consideration of the mechanics of ascent and melt extraction along with phase equilibria clearly shows that partial melting of quartz eclogite best satisfies the chemical constraints of major trace and ree characteristics of high alumina arc basalts

lead isotopic study of early proterozoic wopmay orogen nw canada role of continental crust in arc magmatism lead isotopes are sensitive indicators of crustal interaction in arc magmatism because of the large contrast in lead concentration between mantle derived magmas and continental crust and the three independent decay paths in the u th pb system preserved within wopmay orogen 1 94 to 1 84 ga are the remnants of three magmatic arcs the isotopic composition of lead in alkali feldspars separated from a diverse suite of samples from all three arcs fall in a relatively tight cluster with 206 pb 204 pb 15 59 15 88 207 pb 204 pb 15 27 15 36 and 208 pb 204 pb 35 15 35 54 except for samples from the only plutonic suite within wopmay orogen observed to intrude archean crust plutons intruding archean crust have 206 pb 204 pb 15 29 15 49 206 pb 204 pb 15 23 15 31 and 208 pb 204 pb 35 21 35 30 lead isotopic compositions do not correlate with rock composition both diorites and peraluminous granites have the same lead isotopic composition this isotopic composition