u series constraints on intraplate basaltic magmatism intraplate magmatism represents approximately one tenth of the flux of magma to the earth s surface sleep 1990 this type of magmatism has received considerable attention from petrologists and geochemists as it generally exhibits a wider range of chemical compositions than the more uniform mid ocean ridge basalts hence it is rather paradoxical that our understanding of intraplate magmatism is rather poor in this chapter we review the insights that have been gained from using u series measurements combined with other chemical and isotopic constraints to better understand the sources and processes related to intraplate volcanism several unique constraints can be obtained from measurement of u series disequilibria in basalts first u series fractionation can tell us about the residual phases present during melting as small differences in partitioning behavior between the nuclides will induce distinct signatures second as has been shown by the earlier work of allegre and condomines 1982 th isotope ratios can be used to infer the th u ratio of the mantle source providing another useful probe for mapping mantle heterogeneities lastly as detailed below the time dependence of u series fractionation during melting and melt migration can place constraints on several rate dependent parameters such as the melt production rate and melt velocities an important feature of hotspot magmatism is that in many cases the timing of hotspot activity seems to be decoupled from the motion of the lithospheric plate this observation which has been the basis for proposing the existence of mantle plumes suggests that magmas erupted at hotspots should reveal something about the nature of the deeper mantle understanding the processes of hotspot magmatism should also tell us about the nature of convective motion responsible for hotspots in the following section we first review some of the outstanding issues that need to be resolved to better understand intraplate magmatism we then

comparison of 230th 238u disequilibrium systematics in lavas from three hot spot regions hawaii prince edward and samoa 230th 238u disequilibrium systematics reveal several important characteristics of the mantle source regions and petrogenesis of volcanic rocks in the presumed hot spots of hawaii marion island prince edward hot spot and samoa the 230th232th activity ratios of lavas from these three hot spots 1 06 0 07 1 04 0 08 and 0 81 0 06 respectively imply that the source regions are each nearly homogeneous with thu weight ratios of 2 9 3 0 and 3 8 for marion island and mauna kea hawaii negligible secular variation occurs in the 230th232th initial ratios this supports other evidence for very short transfer time between source and surface significant residence time at depth prior to eruption cannot be ruled out for the samoan lavas we have studied however the data for one of these flows deviate from the proposed 230th232th 87sr86sr correlation condomineset al 1981a in the opposite sense from that expected for such residence if it is assumed that the measured 230th232th ratios of the young lavas reflect thu in their mantle sources then the observed variations among these three hot spots combined with those reported by other workers for iceland the azores and tristan de cunha suggest that these sources are characterized by thu ratios ranging from values similar to that of morb source 2 5 to values similar to those of bulk earth 3 8 mixing of different proportions of depleted and enriched mantle may be responsible for the observed range

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

230th 238u radioactive disequilibria in tholeiites from the famous zone mid atlantic ridge 36 50 n th and sr isotopic geochemistry we analyzed u th and230th 232th activity ratios for a few tholeiites from the mid atlantic ridge famous zone at 36 50 n the results show a fairly wide scatter for both th u and 230th 232th ratios seawater contamination appears to be responsible for this scatter and for the uranium produces an increase in content yielding a 234u 238u ratio greater than 1 and for the th an increase of the 230th 232th ratio which is a very sensitive indicator for contamination also the latter often is selective u th and sr are not affected in the same manner when discarding all data for contaminated samples the famous zone appears to be very homogeneous with ath u ratio value of 3 05 and a 230th 232th ratio value of 1 24 comparison with other active volcanic areas reveals a negative correlation between 230th 232th and87sr 86sr ratios for present lavas which is indicative of a consistency in th u and rb sr fractionation in the source regions of these magmas the th isotopic geochemistry can thus provide useful information for the study of present volcanism information as valuable as that from sr pb or nd isotopes

effect of shallow level contamination on the helium isotope systematics of ocean island lavas the bimodal distribution of helium isotope ratios in ocean island lavas has provided powerful constraints on the composition and evolution of the earth s mantle1 3 high 3he hotspot ratios 3he 4he greater than in mid ocean ridge basalt morb found on hawaii4 and iceland5 are thought to trace pristine plumes from the deep er mantle whereas low 3he hotspot values 3he 3he morb value at tristan da cunha6 and st helena7 are considered to characterize plumes composed in part of recycled oceanic or continental crust here we report the observation of both high 3he and 6low 3he characteristics8 in lavas from a single ocean island heard island in the indian ocean whereas the high 3he lavas provide unambiguous evidence for the involvement of a deep seated plume in their genesis we argue that the low 3he 4he ratios in other lavas result from shallow level contamination by radiogenic helium before eruption these observations call into question the presumed association between low 3he ratios at heard island and elsewhere and ancient crustal material recycled back into the mantle

226 ra excesses in mid ocean ridge basalts and mantle melting the discovery of radioactive disequilibrium among isotopes of the 238u decay chain in recent volcanic rocks1 7 has enhanced our understanding of both the timescale and processes of melting in the mantle excesses of 230th half life 75 200 yr relative to its parent 234u in some of these rocks indicate that thorium is preferentially partitioned into the melt phase and constrain the combined duration of melting and magma transport to less than a few hundred thousand years recent observations of 226ra and 228ra excesses over their respective parents 230th and 228th in individual subaerial volcanoes have led to even more precise determinations of the timescale of magmatic processes3 6 here we report the observation and implications of 226ra half life 1 620 yr excesses in mid ocean ridge basalts these excesses are probably produced during melting in the mantle and the timescale they impose suggests that they are controlled by kinetic processes rather than chemical equilibrium regardless of the process the 226ra excesses constrain the length of the magma transfer times from melting through eruption and sample collection to less than about a thousand years

210po 210pb dating of recent volcanic eruptions on the sea floor the globe encircling mid ocean ridge system accounts for most of the earth s volcanism and influences its heat budget morphology and the composition of the oceans it is therefore important to constrain the size frequency and compositional variability of ridge eruptions but this is made difficult by the remoteness of much of the sea floor and the paucity of applicable radioactive chronometers remote monitoring has provided indirect evidence for active ridge volcanism1 3 which in some cases has been strengthened by submersible observations4 5 but it has not been possible to date these eruptions directly here we present a new chronometer based on 210po 210pb radioactive disequilibrium which allows us to date glassy eruption products within a few years of their eruption our first results on lavas from 9 50 n on the east pacific rise confirm that the ridge erupted only months before sample collection and indicate that the eruptions extended over at least one year the chronometer begins with po volatilization during eruption all glasses analysed were 75 100 degassed a crystalline pillow interior was 20 degassed however implying that the flow surface may control po degassing during deep submarine eruptions although the 210po 210pb chronometer is useful for only a short time frame it should prove valuable for dating recent but unobserved submarine eruptions and determining details of eruptive sequences

icelandic coastal sea surface temperature records constructed putting the pulse on air sea climate interactions in the northern north atlantic part i comparison with hadisst1 open ocean surface temperatures and preliminary analysis of long term patterns and anomalies of ssts around iceland abstract a new comprehensive record of long term icelandic sea surface temperature measurements which have been updated and filled in with reference to air temperature records is presented the new sst series reveal important features of the variability of climate in iceland and the northern north atlantic this study documents site histories and possible resulting inconsistencies and biases for example changes in observing sites and instruments a new 119 yr continuous time series for north iceland sst is presented which should prove particularly useful for investigating air sea ice interactions around northern iceland as this is the only part of the country to be regularly engulfed by winter and or spring sea ice it is therefore highly sensitive to climatic change the coastal series correlate well overall with independent hadley centre sea ice and sst dataset version 1 hadisst1 series from the adjacent open ocean mean r 0 59 although correlations are generally higher in summer than winter

231pa and230th chronology of mid ocean ridge basalts mass spectrometric measurements of235u231pa and238u230th disequilibria are used to further constrain processes producing u series disequilibria in young mid ocean ridge basalts morb and to determine eruption ages for samples from 9 10 n east pacific rise and juan de fuca gorda ridges 231pa 235u activity ratios for axial basalts are large and relatively uniform within ridge segments the systematically large231pa 235u 230th 238u and226ra 230th activity ratios in morb are inconsistent with simple equilibrium melting models and crustal contamination or seawater alteration processes more complicated melting processes involving disequilibrium or heterogeneous melting of mantle sources and disequilibrium transport of melts to the crust could produce the large u series disequilibria in morb 231pa and230th model ages were determined by estimating initial231pa 235u and230th 232th ratios from axial samples and by assuming constancy of these ratios over time all231pa and230th ages agree to 0 7 ka for samples with ages of 0 130 ka these concordant ages validate the model assumptions of the two dating methods and suggest that mantle sources and melt generation processes have been uniform for these areas during the past 130 ka consequently 231pa and230th age dates can be used to quantify the temporal and spatial dependence of volcanism at oceanic spreading centers during the past 375 ka

230th 238u disequilibrium systematics in oceanic tholeiites from 21 n on the east pacific rise significant disequilibrium occurs between 230 th and its parent 238 u in a suite of fresh basalt glasses from the rise project study area at 21 n on the east pacific rise the 230 th 232 th activity ratios observed for eight of nine samples from the crest of the axis at this site are constant within analytical uncertainty with a value of 1 22 this observed homogeneity of 230 th 232 th has two possible interpretations first the measured 230 th 232 th can be considered to indicate a mantle source for the rise basalts with th u of 2 5 this interpretation however conflicts with the proposed correlation between 230 th 232 th and 87 sr 86 sr 1 which predicts that 230 th 232 th should equal 1 33 at the rise site a second possible interpretation is that radioactive decay of 230 th in the basalts themselves or in a magma chamber has decreased 230 th 232 th from 1 33 to the observed values the required time span is 11 000 to 100 000 years however petrologic arguments rule against long residence time in a magma chamber and the spreading rate of this section of the east pacific rise 6 cm yr predicts that the maximum age for axis basalts is 27 000 years both interpretations of the measured 230 th 232 th imply a low th u ratio for the rise basalt source and suggest that the morb source at this location is depleted in th with respect to u relative to primitive mantle or bulk earth in spite of their constant 230 th 232 th the basalts from 21 n have wide ranges of measured th u and thorium and uranium concentrations in apparent conflict with the common assumption that these two elements have very small similar crystal liquid partition coefficients participation of an accessory phase with high th and u concentrations during partial melting or fractional crystallization appears to be required to explain this anomaly major and other trace element compositions provide more information about petrogenetic processes although fractional crystallization can explain the variations in major element composition a more complex process such as continuous melting is required to account for the observed trends in trace element compositions in addition to its use as a tracer the 230 th 238 u disequilibrium technique is capable of putting some constraints on the timing of fractionation events and it may eventually lead to a method for dating very young mid ocean ridge basalts

u series disequilibria in morb from the garrett transform and implications for mantle melting abstract here we report 238u 230th 231pa 226ra disequilibria and 87sr 86sr measurements in 11 mid ocean ridge basalt morb glasses from the garrett transform 13 30 s latitude on the east pacific rise epr whose compositions range from primitive depleted high mgo basalts to evolved basalts u and th concentrations of samples range between 3 and 75 ppb and between 6 and 220 ppb respectively with a corresponding large variation in th u 1 5 2 9 230th 232th varies from 1 2 to 1 6 such that 230th 238u range from 15 excess 230th in a high th u evolved sample to 25 excess 238u in a high mgo sample with low th u out of 11 samples 7 have 238u excess an unusual feature for morb all samples have 226ra excesses with 226ra 230th varying between 1 3 and 3 8 constraining ages since eruption to comparing the garrett transform to the siqueiros transform shows a remarkable correspondence between sample setting composition and disequilibria systematics both settings produce linear trends of 230th 238u as a function of th u consistent with mixing between two melts derived from different depths in the melting column the mixing relationships are identical in both locations the most incompatible rich samples with the highest th u and 230th excess come from the ridge transform intersection rti whereas the most incompatible element poor basalts with the lowest 230th 238u and th u are erupted along leaky transform faults samples with intermediate th u and 230th 238u all come from within intra transform spreading centers consistent with the spreading centers acting to homogenize these diverse magmas the cause of variation in th u could reflect either melting processes or different long lived sources no clear indication exists within these data

uranium and 10 be enrichments by fluids in andean arc magmas volcanic rocks from subduction settings frequently contain 10be enrichments that have been attributed to sediment subduction1 6 and are often characterized by 238u 230th ratios greater than unity in contrast to mid ocean ridge and ocean island basalts7 8 here we report such an excess of 238u over 230th in historic lavas from the southern volcanic zone of chile 33 42 s the greatest uranium excess is found south of 39 s where 10be 9 be ratios are highest and the trench is filled with sediment these features suggest that both u and 10be enrichments result from preferential partitioning of u and be into a fluid phase during dehydration of the recently subducted sediments the presence of strong 238u 230th disequilibria in basaltic magmas of the southern volcanic zone suggests that the timescale for dehydration melting and eruption in this arc is probably less than 20 000 yr

238u 230th disequilibria and mantle melting processes a discussion abstract this paper discusses the implications of 238 u 230 th disequilibria in morb and oib on mantle melting it is shown that melting rates can only be calculated if the melting process is not batch melting at present however the uncertainties in u and th partition coefficients are such that in most cases batch melting cannot be discarded moreover a simple argument based on the th sr isotope correlation suggests that batch melting might best approximate the actual melting process in the mantle at least in the case of oib the existence of mantle heterogeneities e g the marble cake model of allegre and turcotte allegre c j turcotte d 1986 implications of a two component marble cake mantle nature 323 123 127 when taken into account suggests that complex melting models might not be necessary to interpret 238 u 230 th data in particular preferential melting of garnet pyroxenites could explain the origin of oib whereas morb could derive from larger degrees of melting of garnet lherzolites however the large 231 pa and 226 ra excesses found in morb are better explained by dynamic melting models if they are actually produced through crystal melt partitioning during mantle melting in any case it is worth emphasizing that derivation of melting rates is strongly model dependent and should be taken with caution

investigating solid mantle upwelling beneath mid ocean ridges using u series disequilibria ii a local study at 33 s mid atlantic ridge abstract uranium series disequilibria have been used to study the physical melting regime along a section of the southern mid atlantic ridge where the largest mantle bouguer anomaly yet observed on ridges exists zero age excesses for 230th 238u and 231pa 235u are age constrained by the presence of measured 226ra 230th disequilibria by this criterion 7 out of 15 samples are younger than 8000 years zero age 230th 238u range between 1 06 and 1 25 while zero age 231pa 235u range between 1 77 and 3 57 concentrations of th range from 100 to 866 ppb and strongly correlate with k ti the u series data differentiate the segment with the large gravity anomaly from the surrounding segments specifically this segment shows combined 231pa and 230th systematics unlike any observed in morb thus far having exceptionally low zero age 231pa excesses the u series data can be modeled using upwelling rates similar to the half spreading rate of this area of ridge the disequilibria systematics for the anomalous segment 4 possibly suggest a solid upwelling rate that is a factor of two higher than the adjacent segments the similarity between melting at segment 4 and at ocean islands is noted

two mantle domains and the time scales of fluid transfer beneath the vanuatu arc u th isotope disequilibria can provide constraints on the time elapsed since fluid addition to the mantle wedge beneath island arcs the vanuatu arc offers new insights into these processes because pb isotopes there are not dominated by components from the subducting plate and so preserve the signatures of the mantle wedge the pb isotope data document the presence of separate pacific and indian mantle domains beneath the arc volcanoes the indian mantle was brought beneath the central part of the arc from the backarc by collision with the d entrecasteaux ridge resulting in a slowing of subduction there the distinction in the mantle wedge composition is also uniquely apparent in u th isotope data which define two subparallel arrays on the u th equiline diagram one anchored to high u th pacific matle and the other to lower u th indian mantle these data provide clrear evidence of the effects of variaible mantle composition on u th isotope disequilibria we argue that such arrays faithfully record the time elapsed since fluid release from the subducting plate the data indicate that this occurred ca 16 ka in the area of collision and slow subduction but ca 60 ka where the rate of subduction is substantially faster this suggests a link between the rate of subduction and the time elapsed since fluid release

time scales of magma formation ascent and storage beneath subduction zone volcanoes there is now sucient information to attempt an integrated model for melt gener ation transfer and storage beneath subduction zone volcanoes fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr to as little as less than 1 kyr before eruption this supports models in which uid addition is closely linked to partial melting though there may also be evidence for a component of decompression melting the timing of the onset of uid addition may be linked to the rate of subduction i e water supply rate and the angle of subduction and consequently the thermal structure of the mantle wedge in contrast contributions from subducted sediments to subduction zone lava sources appear to occur some 350 kyr 4 myr before eruption evidence for partial melting of the sediment component combined with the short uid transfer times phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate new 226 ra data permit only a short period of time between uid addition and eruption this requires rapid melt segregation magma ascent by channelled ow and minimal residence time within the lithosphere typically the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs inferred from some geophysical data to lie within the lithospheric mantle mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers where they can evolve to dacitic compositions via fractionation typically combined with assimilation on time scales of a few thousand years or less

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

ridge hotspot interaction along the mid atlantic ridge between 37 30 and 40 30 n the uth disequilibrium evidence abstract this study reports u th disequilibrium data obtained by mass spectrometry for basaltic glasses collected along the azores platform portion of the mid atlantic ridge 37 30 40 30 n a region characterized by both a geochemical and bathymetric gradient high th and u concentrations as well as th u ratios document an enriched geochemical signature 230 th 238 u activity ratios range from 1 20 to 1 35 and are thus systematically larger than most epr morbs reported in the literature 230 th 232 th activity ratios show remarkable homogeneity for multiple samples taken from single dredge hauls additionally samples with the highest th concentrations 2 4 ppm have among the highest th isotope ratios taken together these observations rule out assimilation of 230 th rich sediment as an explanation for the 230 th 238 u systematics the relatively large 230 th excesses in the erupted lavas may be related to the influence of the enriched azores mantle plume source the lack of observed correlations between 230 th excess and trace element and isotopic indices of source enrichment however rules out source composition as an explanation for the variations in 230 th 238 u excess 230 th is correlated with the axial depth of the ridge in the study area with the shallowest portions showing the largest extents of disequilibrium this may reflect more melting in the presence of garnet for the shallow segments and suggests that melting begins well within the garnet peridotite stability field 35 kbar in the mantle beneath the azores segment of the mar at the ridge segment scale 230 th excesses tend to be smaller near segment boundaries this could reflect differences in the melting process or less frequent magmatism in these zones these results demonstrate the potential for u th systematics to constrain the depth and degree of melting as well as the rate of mantle upwelling even in the presence of source chemical heterogeneity

constraints on melt movement beneath the east pacific rise from 230th 238u disequilibrium we report 230th 238u disequilibrium data on mid ocean ridge basalts recovered 5 to 40 kilometers off the ridge axis near 9 30 n of the east pacific rise these data indicate near symmetrical eruptions of normal mid ocean ridge basalts nmorbs and incompatible element enriched mid ocean ridge basalts emorbs as far as 20 kilometers off axis our results suggest large scale subsurface lateral transport of nmorb melt at 19 to 21 centimeters per year and also provide constraints on the petrogenesis of emorbs of off axis origin

effects of partial melting on the uranium decay series distinction between possible partial melting processes using stable incompatible trace elements depends largely on the mass fraction of liquid produced at low degrees of melting less than 5 both incompatible element concentrations and interelement fractionations are greater for batch fractional and accumulated fractional melting than for dynamic or continuous melting at higher degrees of melting uncertainty in initial source composition and small fractionations between incompatible elements make it unlikely that the most realistic melting models batch and dynamic could be discriminated however it is possible for these melting processes to be distinguished by the magnitude of radioactive disequilibria that is produced within the 238u decay series even for durations of partial melting much longer than the half lives of 230th and 226ra both230th 238u and 226ra 230th disequilibria can occur because of differences in the bulk solid liquid partition coefficients for these elements the magnitude of these disequilibria depends on melting process the 230th 232th of a dynamic melt can differ significantly from that of a batch melt and will not necessarily indicate the original 238u 232th of the source if morb is formed by a dynamic melting process then the th u of the upper mantle near ridge axes may be higher than that calculated from the 230th 232th of the basalts

238u 230th 226ra fractionation in historical lavas from the azores long lived source heterogeneity vs metasomatism fingerprints abstract this study reports 238u 230th 226ra nuclides measured by mass spectrometry trace elements and 87sr 86sr isotopes on historical alkali basalts from the azores region the 230th 232th activity in the alkali basalts spans a range from 0 897 in sao miguel to 1 382 in terceira island that is almost as large as the global variation of mid ocean ridge basalts morb and oceanic island basalts oib the 230th 238u activity 10 40 th u 2 92 3 74 and the 87sr 86sr 0 70354 0 70467 also display large variations the 226ra 230th activity varies from 0 84 in sao miguel to 1 60 in pico island samples from pico faial and terceira islands are found to plot on the negative 230th 238u vs 226ra 230th correlation defined by the 1730 1736 eruption of lanzarote canary islands sigmarsson et al earth planet sci lett 162 1998 137 except for the sample from sao miguel island we propose that the negative correlation reflects a mixing between two end member melts one displaying a low 230th 238u and a high 226ra 230th and the other end member displaying a high 230th 238u and a low 226ra 230th numerical model suggests that the high 226ra low 230th excesses end member represented by samples from pico island can be explained by partial melting of a garnet lherzolite the other end member represented by the terceira lava is shown to result from the interaction of low degree melts with the metasomatized mantle containing phlogopite phlogopite crystals in contact with a melt enriched in 226ra and ba have a 226ra 230th 1 and high ba th because of crystal affinities for ra and ba relative to th and u after 8000 years however the unsupported 226ra excess will have decreased down to 0 as phlogopite displays little amount of u and th so during the process of interaction with phlogopite crystals low degree melts formed in the garnet stability field that are characterized by high 230th 238u 226ra 230th and ba th will loose the 226ra excess scavenged by phlogopite while the 230th 238u and ba th will remain unaffected one finds that this process may be valid over 250 years maximum so that only the first interacting melts may preserve a high 230th 238u low 226ra 230th activity on sao miguel island it is shown that the low 230th 238u while the source is clearly enriched in th cannot be explained by a lack of garnet in the mantle neither by a higher degree of melting the 226ra deficit 15 and low ba th and sr th are explained by partial melting of an enriched mantle in the presence of phlogopite

uzth isotopic systematics at 13 n east pacific ridge segment fresh basaltic glasses have been analyzed for uzth disequilibrium systematics as part of an extensive study on the east pacific rise epr at 12 45 n these samples are well described in terms of major and trace elements as well as in nd pb and sr isotopes our results show significant heterogeneities in the mantle source at a small scale and show heterogeneities at larger scales also when compared to other epr data u and th concentration and isotopic data rule out fractional crystallization as a main process and support a mixing model in agreement with the marble cake model developed by alle gre and turcotte and constrained by trace elements and nd sr and pb isotopes on the same samples by prinzhofer et al based on the high 230th 232th isotopic ratios on recent tholeiites especially the th u values inferred for their sources clearly show that the upper mantle th u has decreased with time

off axis volcanism at the east pacific rise detected by uranium series dating of basalts recent detailed surveys of the east pacific rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast spreading ocean ridge crests1 7 in parallel with geological and geochemical investigations it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium series disequilibria methods8 11 here we use 238u 230th and235u 231pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9 31 n on the east pacific rise most of the ages are younger than would be expected based on off axis distance and spreading rate we infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0 5 2 km outside the axial summit caldera with some volcanism occurring as far as 4 km off axis melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas

bayesian flood frequency analysis with paleohydrologic bound data 1 it is valuable to construct likelihood functions that rigorously incorporate measurement errors and annual peak discharge historical and paleohydrologic bound information in bayesian flood frequency analyses estimates of primary posterior modes for common three parameter frequency distributions are constructed using simulated annealing and the simplex method parameter and flood frequency probability intervals are calculated directly by systematic parameter space integration bayesian flood frequency analyses with annual peak discharge historical and paleohydrologic bound data for the santa ynez river california and the big lost river idaho demonstrate that paleohydrologic bounds reduce quantile biases by placing large observed peak discharges in their proper long term contexts and substantially narrow peak discharge confidence intervals when estimating floods with low exceedance probabilities

usercalc a web based uranium series calculator for magma migration problems 1 measured departures from secular equilibrium in the uranium series decay chains provide important constraints on the rates of recent mantle and crustal processes nevertheless the inferences drawn from these observations depend upon the models used to interpret them while several useful models for u series exist they are not all equally easy to use and it is often the ease of use rather than the utility of the model that determines which models are applied the purpose of this paper is to level the playing field by making some of the more general u series transport models accessible to a wider community through the usercalc web site available from lamont doherty earth observatory at www ldeo columbia edu mspieg usercalc these models calculate the effects of both melting and melt transport on two u series chains 238u 230th 226ra and 235u 231pa and allow the user to input their own petrological insight and data to explore the behavior of this model the purpose of the site is to provide a tool that can spur additional collaborations between geophysicists and geochemists as well as drive more rapid exploration evolution of u series models

volcanic activity and crust mantle exposure in the ultrafast garrett transform fault near 13 28 s in the pacific the 130 km long garrett transform fault which offsets two ultrafast 14 5 cm yr epr spreading segments is the site of volcanic and tectonic activity evidence for recent volcanic events was observed during several dives on oblique ridges 3100 3500 m high trending 040 alpha beta and gamma and on small constructional mounds located in the central transform basin and eastern valley at a depth of 3500 5000 m the constructional features show a fissured terrain suggesting a component of lithospheric opening the strike slip motion of the transform is inferred at several locations between the two epr transform intersection zones from the presence of abundant sediment free talus with crushed rock debris fresh scarps and well developed striations on outcrops of serpentinized peridotites and metamorphosed volcanic breccia the width of the active tectonic zone varies between 200 and 3000 m in the explored area near the eastern and western epr transform intersection zones along the e w trending median ridge walls in the eastern and western transform troughs of the central basin and at several sites on the south facing wall of the transform domain first escarpment the presence of recent volcanic activity implies that the garrett transform behaves as a leaky transform fault the most common volcanics consisting of plagioclase and olivine phyric lava erupted in the transform are compositionally more depleted k ti 0 08 and less evolved mg 60 70 than the basalts encountered on the southern epr segments mg 40 66 and k ti 0 05 0 25 this suggests that different magmatic conditions are operating in the transform domain the transform strike slip motion is responsible for the emplacement of a sliver of serpentinized peridotite located in an area about 2 km wide and 10 km long along the median ridge at the contact with outcrops of metamorphosed basaltic breccia the ultramafics consist of residual harzburgites often impregnated with irregular lenses and dykes of basaltic liquids plagioclase wehrlite and gabbro and are found associated with troctolites dunite and gabbros the emplacement of the ultramafics and their gabbroic inclusions has resulted in the presence of metamorphosed tectonic breccia forming part of the median ridge in the central transform basin