luminescence dating of the loess profile at dolni vestonice czech republic abstract thermoluminescence tl and infra red stimulated luminescence irsl studies were carried out on 21 samples from loess profiles at dolni vestonice the irsl measurements gave systematically lower equivalent doses eds than the tl measurements this could be due to inadequate preheat being applied to the former dose rate intercomparisons confirm the view that alpha counting should be carried out on finely ground material comparison of ed determination by regeneration and additive dose methods did not suggest sensitivity increases on bleaching but for the older samples the tl ages were underestimated compared with the expected ages based on the correlation of palaeosol stratigraphy with the framework provided by the deep sea oxygen isotope record using the hypothesis of luminescence decay with a lifetime of 150 ka suggests that the previous ages assigned to the palaeosols may not be correct

age and petrology of alkalic postshield and rejuvenated stage lava from kauai hawaii at the top of the waimea canyon basalt on the island of kauai rare flows of alkalic postshield stage hawaiite and mugearite overlie tholeiitic flows of the shield stage these postshield stage flows are 3 92 ma and provide a younger limit for the age of the tholeiitic shield stage the younger koloa volcanics consist of widespread alkalic rejuvenated stage flows and vents of alkalic basalt basanite nephelinite and nepheline melilitite that erupted between 3 65 and 0 52 ma all the flows older than 1 7 ma occur in the west northwestern half of the island and all the flows younger than 1 5 ma occur in the east southeastern half the lithologies have no spatial or chronological pattern the flows of the koloa volcanics are near primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet bearing source that has about two thirds the concentration of p2o5 rare earth elements and sr of the source of the honolulu volcanics on the island of oahu the same lithology in the koloa and honolulu volcanics is generated by similar degrees of partial melting of distinct source compositions the lavas of the koloa volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt respectively phases that remain in the residue of the honolulu volcanics such as rutile and phlogopite are exhausted during formation of the koloa volcanics at all but the smallest degrees of partial melting the mantle source for kauai lava becomes systematically more depleted in 87sr 86sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage at the same time the lavas become systematically more enriched in incompatible trace elements on a shorter timescale the lavas of the koloa volcanics display the same compositional trends but at a lower rate of change the source characteristics of the koloa volcanics considered along with those of the honolulu volcanics support a mixing model in which the source of rejuvenated stage lava represents large percent melts of a plume source mixed with small amounts of small percent melts of a heterogeneous mid ocean ridge source

isotopic evolution of lavas from haleakala crater hawaii pb and sr isotopic ratios have been determined for tholeiitic shield building alkalic cap and post erosional stage lavas from haleakala crater pb isotopic compositions of the tholeiites overlap those of the alkalic cap lavas although87sr 86sr ratios of these two suites are distinct alkalic cap and post erosional lavas appear to be indistinguishable on the basis of sr and pb isotopic composition sr and pb isotopic ratios of haleakala post shield building lavas are positively correlated such a trend is previously undocumented for any suite of hawaiian lavas and contrasts with the general negative correlation observed for data from hawaiian tholeiites these relations are consistent with a three component petrogenetic mixing model specifically it is proposed that magma batches at individual hawaiian volcanoes formed by 1 mixing of melts generated from mantle plumes containing two isotopically distinct mantle components primitive vs enriched and 2 subsequent variable degrees of interaction between these plume melts and a third morb signature mantle reservoir prior to their emplacement in a crustal magma chamber these observations and inferences provide new constraints on physical models of hawaiian magmatism based on observed temporal isotopic variations of haleakala lavas it is suggested that the ratio of enriched primitive mantle components in the hawaiian plume source decreases during the waning stages of alkalic volcanism over the same time interval both decreasing melt production and protracted residence of ascending melts within the upper mantle contribute to a systematic increase in the ratio of depleted vs plume component

role of olivine cumulates in destabilizing the flanks of hawaiian volcanoes the south flank of kilauea volcano is unstable and has the structure of a huge landslide it is one of at least 17 enormous catastrophic landslides shed from the hawaiian islands mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments slip on a low friction decollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest or why this section contains a 300 km3 aseismic volume above the seismically defined decollement it is proposed that this aseismic volume adjacent to the caldera in the direction of flank slip consists of olivine cumulates that creep outward pushing the south flank seawards average primary kilauea tholeiitic magma contains about 16 5 wt mgo compared with an average 10 wt mgo for erupted subaerial and submarine basalts this difference requires fractionation of 17 wt 14 vol olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates submarine erupted kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas the deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed probably with as little as 5 10 vol intercumulus liquid before entrainment of the xenocrysts the olivine cumulates were at magmatic temperatures 1100 c when the xenocrysts were entrained olivine at 1100 c has a rheology similar to ice and the olivine cumulates should flow down and away from the summit of the volcano flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank leading to an extensional region that localizes deep intrusions behind the flank these intrusions add to the seaward push this mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and therefore limits catastrophic sector failure of hawaiian volcanoes to their active growth phase when the core of olivine cumulates is still hot enough to flow

volcano spacing and plate rigidity in plane stresses which accompany the flexural deformation of the lithosphere under the load adjacent volcanoes may govern the spacing of volcanoes in hotspot provinces specifically compressive stresses in the vicinity of a volcano prevent new upwelling in this area forcing a new volcano to develop at a minimum distance that is equal to the distance in which the radial stresses change from compressional to tensile the inflection point if a volcano is modeled as a point load on a thin elastic plate then the distance to the inflection point is proportional to the thickness of the plate to the power of 3 4 compilation of volcano spacing in seven volcanic groups in east africa and seven volcanic groups of oceanic hotspots shows significant correlation with the elastic thickness of the plate and matches the calculated distance to the inflection point in contrast volcano spacing in island arcs and over subduction zones is fairly uniform and is much larger than predicted by the distance to the inflection point reflecting differences in the geometry of the source and the upwelling areas

east molokai and other kea trend volcanoes magmatic processes and sources as they migrate away from the hawaiian hot spot 1 there are geochemical differences between shield lavas from the two parallel trends kea and loa defined by young hawaiian volcanoes the shield of east molokai volcano at greater than 1 5 ma is the oldest volcano on the kea trend sequences of older tholeiitic to younger alkalic basalt that erupted as this volcano evolved from the shield to postshield stage of volcanism are well exposed much younger 0 34 0 57 ma alkalic basalt and basanite erupted during rejuvenated stage volcanism like rejuvenated stage lavas erupted at other hawaiian volcanoes rejuvenated stage east molokai lavas have relatively low 87sr 86sr and high 143nd 144nd such ratios reflect a source component with a long term depletion in abundance of incompatible elements on the basis of positive correlations of 87sr 86sr versus 206pb 204pb and negative correlations of these isotopic ratios with nb zr a smaller proportion of this depleted component also contributed to the late shield postshield lavas erupted at east molokai and the other kea trend volcanoes haleakala and mauna kea at each of these kea trend volcanoes as the volcano moved away from the hot spot the extent of melting and magma supply from the mantle decreased the depth of melt segregation increased and there was an increasing role for a component with long term relative depletion in incompatible elements this depleted component has kea trend pb isotopic characteristics and relatively low 208pb 204pb at a given 206pb 204pb and it is probably not related to oceanic lithosphere or the source of mid ocean ridge basalt the overlap in sr nd and pb isotope ratios of recent kilauea shield lavas and 550 ka mauna kea shield lavas has been used to argue that kea trend shield volcanism samples a vertically continuous geochemically distinct stripe which persisted in the hot spot source for 550 kyr eisele et al 2003 abouchami et al 2005 as kea trend volcanoes migrate away from the hot spot and evolve from the shield to postshield stage there are systematic changes in sr nd and pb isotope ratios however the overlap of sr nd and pb isotope ratios in late shield postshield lavas from mauna kea 350 ka and east molokai 1 5 ma show that the periphery of the hot spot sampled by kea trend postshield lavas also had long term geochemical homogeneity

solitary waves in mantle plumes numerical calculations in two dimensions show that solitary wave disturbances can propagate along thermal plumes in a homogeneous viscous fluid with a thermally activated rheology similar to the mantle comparison of the numerical results with analytic results from an idealized two fluid model indicates that the two fluid model can be applied to solitary waves in the thermally activated plume a two fluid model of solitary waves on cylindrical conduits is accordingly applied to mantle plumes to estimate the propagation speeds time durations and pulse lengths of solitary waves in mantle plumes as a function of background mantle viscosity plume flux plume density deficit plume viscosity and the volume of material transported by the solitary wave it is inferred that mantle plumes may be episodic via solitary waves and that these disturbances might account for observed 0 10 m y variations in the durations and spacings of episodes of enhanced hotspot volcanic activity solitary waves in mantle plumes could be generated by interactions among plumes and between plumes and the large scale time dependent mantle circulation

rejuvenated stage volcanism after 0 6 m y quiescence at west maui volcano hawaii new evidence from k ar ages and chemistry of lahaina volcanics abstract west maui s rejuvenated stage lahaina volcanics were erupted from four discrete sites new k ar ages indicate two pulses of volcanism the older about 0 6 ma and the younger about 0 4 ma compositionally the lava flows are entirely basanitic but each pulse is diverse the underlying postshield stage honolua volcanics were emplaced by about 1 2 ma on the basis of previously published ages therefore the duration of volcanic quiescence prior to rejuvenation is about 0 6 m y at west maui much longer than estimated previously

evidence for two shield volcanoes exposed on the island of kauai hawaii the island of kauai has always been interpreted as a single shield volcano but lavas of previously correlated reversed to normal magnetic polarity transitions on opposite sides of the island differ significantly in isotopic composition samples from west kauai have 87sr 86sr 18 25 samples from east kauai have 87sr 86sr 0 7037 nd 6 14 and 206pb 204pb 18 25 available data suggest that a younger eastern shield grew on the collapsed flank of an older western one

melting in the hawaiian plume at 1 2 ma as recorded at maui nui the role of eclogite peridotite and source mixing 1 the volcanoes of maui nui west moloka i east moloka i lana i west maui haleakala and kaho olawe record hawaiian magmatism at 1 2 ma lavas from these volcanoes nearly span the compositional range erupted from all the hawaiian volcanoes over the past 5 myr and represent both the kea and ko olau compositional end members of hawaiian lavas many aspects of major and trace element and isotope compositions of hawaiian shield stage lavas are consistent with ancient recycled oceanic lithosphere in the plume sources of kea and ko olau type magmas lassiter and hauri 1998 blichert toft et al 1999 hypotheses that describe the compositional range of hawaiian lavas as originating from ancient oceanic lithosphere in the hawaiian plume implicitly or explicitly infer lithologic heterogeneity in the plume we present trace element models for the origin of these end members that explicitly address the petrologic complexities of melting eclogite derived from ancient oceanic lithosphere in the plume trace element la nb sm yb sm hf and sm nb major element and isotope compositions of lana i which erupts dominantly ko olau type lavas are consistent with the origin of these lavas in large degree 60 70 melts of ancient upper oceanic crust basalt sediment that mix with plume derived haleakala type melts trace element sm yb hf zr and hf nb and isotope compositions of west maui and east moloka i which erupt dominantly kea type magmas are consistent with an origin in ancient depleted oceanic lithosphere that has been refertilized with moderate degree melts 10 30 of associated crustal gabbro the physical mechanisms melt melt versus melt solid mixing through which the oceanic crustal components melt and mix within the plume lead to the generation of isotopically homogeneous kea type lavas and isotopically heterogeneous ko olau type lavas the volcanoes of maui nui record the exhaustion of the ko olau component and the initiation of the kea component as dominant compositional end members in the hawaiian plume

the anatomy of a carboniferous epithermal ore shoot at pajingo queensland setting zoning alteration and fluid conditions the paleozoic scott lode gold silver epithermal deposit situated at pajingo northeast queensland is hosted by feldspathic volcanolithic sandstones quartzose sandstones ignimbrites andesitic voltanits with interbedded block and ash deposits lapilli tuffs and andesitic to dioritic intrusive rocks the scott lode deposit contains 1 23 million metric tons of 9 9 g t gold and 38 9 g t silver there are six principal types of alteration assemblages which are zoned with respect to the main vein structure propylitic potassic adularia intermediate argillic silicic kaolinitic and ferroan carbonate overprinting textural relationships indicate that the propylitic type is earliest progressively followed by potassic adularia then intermediate argillic and finally silicic alteration auriferous veins are largely coeval with the silicic and intermediate argillic alteration types kaolinitc and ferroan carbonate occur as late stage alteration products postdating the precious metal mineralization principal ore and sulfide minerals include pyrite sphalerite galena chalcopyrite hessitc tennantite tetrahedrite argentitc petzite electrum and native gold gangue minerals in the veins are quartz illitc adularia calcite and locally kaolinitc the main stage quartz is zoned with respect to mineralogy mineralization styles metal distribution patterns and vein textres three major zones can be recognized zone i an upper precious metal moss chalcedonic crustiform colloform quartz zone zone ii intermediate precious base metal crystalline quartz coarse grained sulfide dominated crustiform colloform quartz zone and zone iiideep sparse base metal weakly banded crystalline comb quartz zone the ore interval occurs in zones i and ii with the transition to zone iii effectively defining the limit of mineralization economic mineralization at the scott lode deposit was laid down from dilute fluids 0 7 2 5 wt nac1 equiv with a very low concentration of co2 homogenization temperatures of 170 o to 315oc exhibit vertical variation with higher temperatures at greater depth the presence of coexisting liquid and gas rich inclusions in mineralized quartz together with adularia vein brecciation and pronounced crustiform colloform banding indicates hydrothermal boiling as the principal precipitation mechanism for gold and silver deposition thermodynamic modeling of adiabatic boiling of a fluid with an initial concentration of 2 to 5 ppb gold at 300oc indicates that such a fluid can deposit ores containing 10 ppm gold and 260 ppm silver at the temperature intervals recorded by the fluid inclusion work

structural and petrologic evolution of the lihue basin and eastern kauai hawaii the topography of the eastern part of the hawaiian island of kauai is dominated by the lihue basin a large 110 km 2 semicircular depression bounded by steep cliffs and partly filled by late rejuvenated stage or posterosional stage volcanic material as with other large semicircular basins on ocean island volcanoes the subsurface geology and origin e g structural collapse vs fluvial erosion of the lihue basin are poorly understood new analyses of samples collected from eastern kauai and drill holes within the basin document several important features of the late stage geologic evolution of kauai first thick 300 m sequences of rejuvenated stage koloa volcanics in the lihue basin show systematic basin wide geochemical trends of increasingly incompatible elements with time indicating a gradual decrease in the extent of partial melting of mantle sources with time second beneath the rejuvenated stage volcanics in the basin a thin layer of postshield alkalic stage lavas e g hawaiites and mugearites overlies older shield stage tholeiitic lavas of the napali member indicating that the lihue basin formed by structural collapse not fluvial erosion third a large 2 5 km 3 matrix supported breccia interpreted as deposits of one or more debris flows is within the rejuvenated stage volcanics throughout the basin and correlates with surficial exposures of the palikea breccia west of the basin isotopic compositions of the bulk breccia are similar to those of tholeiites from the east side of kauai and distinct from those of west kauai tholeiites clasts within the breccia are dominantly hawaiite and alkali gabbro the source region of the breccia in the steep cliffs and highlands of the central massif to the west of the basin must contain magmatic products of an extensive postshield alkalic stage including hawaiite flows and one or more large intrusive bodies or ponded sequences of alkali gabbro

geochemistry of diverse basalt types from loihi seamount hawaii petrogenetic implications the wide variety of basalt types tholeiitic to basanite dredged from loihi seamount have minor and trace element abundances that are characteristic of subaerial hawaiian basalts thereby confirming that loihi seamount is a manifestation of the hawaiian hot spot within the loihi sample suite there are well defined positive correlations among abundances of highly incompatible elements p k rb ba nb light ree and ta and moderately incompatible elements sr ti zr and hf and between mgo ni and cr however within the loihi suite abundance ratios of geochemically similar elements zr hf nb ta and la ce vary by factors of 1 2 1 5 and abundance ratios of highly incompatible elements such as p ce p th k rb ba th and la nb vary by factors of 1 2 2 5 these abundance ratios are not readily changed by different degrees of fractionation and melting therefore we conclude that these samples are not genetically related by different degrees of melting of a compositionally homogeneous source on the basis of k p k ti p ce zr nb th p and la sm abundance ratios the twelve samples studied in detail can be divided into six geochemical groups samples within each group are similar in 87sr 86sr 1 and intra group compositional variations may reflect low pressure fractionation and different degrees of melting in addition crossing chondrite normalized ree patterns within the alkalic basalt groups reflect equilibration of the magmas with garnet in ratio ratio plots involving abundance ratios of highly incompatible elements e g la p nb p k p rb p ba p and th p the geochemical groups define linear arrays suggestive of mixing however these data combined with the isotopic data are not consistent with two component mixing

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

a submarine perspective of the honolulu volcanics oahu lavas and volcaniclastic deposits were observed and collected from 4 submarine cones that are part of the honolulu volcanics on oahu hawaii the locations of these and a few additional but unsampled vents demonstrate that nearly all the vents are located on or very close to the shoreline of oahu with the most distal vent just 12 km offshore the clastic samples and outcrops range from coarse breccias to cross bedded ash deposits and show that explosive volcanism at depths between about 350 and 590 m depth played a part in forming these volcanic cones the eruptive styles appear to be dominantly effusive to strombolian at greater depths but apparently include violent phreatomagmatic explosive activity at the shallower sites along the submarine southwest extension of the koko rift the compositions of the recovered samples are broadly similar to the strongly alkalic subaerial honolulu volcanics lavas but the submarine lavas erupted further from the koolau caldera have slightly more radiogenic sr isotopic ratios and trace element patterns that are distinct from either the subaerial honolulu volcanics or the submarine north arch lavas these patterns are characterized by moderate to strong positive sr and p anomalies and moderate to strong negative cs rb u th zr and hf anomalies most samples have strong negative k and moderate negative ti anomalies as do all subaerial honolulu volcanics and north arch samples but one group of samples from the koko rift lack this chemical signature the data are consistent with more garnet in the source region for the off shore samples than for either the on shore honolulu volcanics lavas new ar ar ages show that eruptions at the submarine vents and diamond head occurred between about 0 5 ma and 0 1 ma with the youngest ages from the koko rift these ages are in general agreement with most published ages for the formation and suggest that some much younger ages reported previously from the koko rift are probably erroneously young d 2005 elsevier b v all rights reserved

ages rare earth element enrichment and petrogenesis of tholeiitic and alkalic basalts from kahoolawe island hawaii kahoolawe island hawaii 18 11 km is a basaltic shield volcano with caldera filling lavas seven identified postshield vents and at least two occurrences of apparent rejuvenated stage eruptive we examined 42 samples that represent all stages of kahoolawe volcano stratigraphy for their petrography whole rock major and trace element contents mineral compositions and k ar ages the two oldest shield samples have an average age of 1 34 0 08 ma and four postshield samples 3 are alkalic average 1 15 0 03 ma ages of 1 08 and 0 99 ma for two additional tholeiitic samples probably are minimum ages whole rock major and trace element and mineral compositions of kahoolawe shield and caldera fill laves are generally similar to the lavas forming kilauea and mauna loa tholeiitic shields but in detail kahoolawe shield lavas have distinctive compositions an unusual aspect of many postshield ka hoolawe lavas is anomalously high ree and y abundances up to 200 ppm la and 175 ppm y and negative ce anomalies these enrichments reflect surficial processes where weathering and soil development promoted ree y transport at the weathering front major element abundances mgo 10 6 wt for shield and caldera fill basalts are consistent with fractionation of ol px pl in frequently replenished magma reservoirs in general tholeiitic basalts erupted from late vents are higher in sio2 than the shield lavas and temporal differences in parental magma compositions are the likely explanation alkalic basalts that erupted from vents are comparable in composition to those at other hawaiian volcanoes trace element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source apparent rejuvenated stage basalts i e emplaced after substantial kahoolawe erosion are tholeiitic unlike the rejuvenated stages at other hawaiian volcanoes alkalic kahoolawe like several other hawaiian volcanoes has intercalated tholeiitic and alkalic basalts in the postshield stage but it is the only volcano that appears to have produced tholeiitic rejuvenated stage lavas

high resolution 40ar 39ar geochronology of the louisville seamounts iodp expedition 330 drill sites implications for the duration of hot spot related volcanism and age progressions

intershield geochemical differences among hawaiian volcanoes implications for source compositions melting process and magma ascent paths as hawaiian volcanoes develop their lavas systematically change in composition and isotopic ratios of sr nd and pb these trends provide important constraints for understanding plume related volcanism as a volcano migrates away from the hotspot there are also geochemical differences between hawaiian shields in particular lavas from adjacent shields such as kilauea and mauna loa on hawaii and koolau and waianae on oahu have significant differences in abundances of some major and incompatible elements and isotopic ratios of sr nd and pb some incompatible element abundance ratios such as zr nb and sr nb are correlated with intershield differences in sr and nd isotope ratios but these isotopic ratios are not correlated with intershield differences in major element composition or even parent daughter abundance ratios such as rb sr and sm nd moreover at kilauea and mauna loa the intershield differences have apparently persisted for a relatively long time perhaps 100 ka these intershield geochemical differences provide important constraints on plume volcanism specifically i each volcano must have distinct magma ascent paths from the region of melt segregation ii the 25 50 km distance between adjacent but geochemically distinct shields requires that the sources vary on a similar scale and that the melt production region is similarly restricted the absence of correlations between lava compositions and radiogenic isotope ratios provides evidence for significant differences in melting process such as each shield forming by a different mean extent of melting with melt segregation at different mean pressures two types of models are consistent with the intershield geochemical differences i a relatively large radius ca 40 km plume conduit with a systematic spatial distribution of geochemical heterogeneities or ii a small radius less than 20 km plume conduit composed of geochemically distinct diapirs because relatively small radius diapirs of limited vertical extent are too small to create the large hawaiian shields a possible alternative is a continuous conduit containing solitary waves which transport geochemically distinct packets of material

population density and zoning of olivine phenocrysts in tholeiites from kauai hawaii the population density of olivine phenocrysts of the tholeiites display an exponential variation which is typical of igneous as well as contact metamorphic rocks the exponential variation is explained by a new growth probability model which is consistent with experimental work the forsterite content of the olivine phenocrysts decreases with decreasing size various phenocryst features suggest that the tholeiites first crystallized slowly in a magma chamber after which they underwent crystallization for a short period of time in a feeder dyke before eruption took place

formation of submarine flat topped volcanic cones in hawai i high resolution bathymetric mapping has shown that submarine flat topped volcanic cones morphologically similar to ones on the deep sea floor and near mid ocean ridges are common on or near submarine rift zones of kilauea kohala or mauna kea mahukona and haleakala volcanoes four flat topped cones on kohala were explored and sampled with the pisces v submersible in october 1998 samples show that flat topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage similarly shaped flat topped cones on the northwest submarine flank of ni ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage submarine postshield stage eruptions on hilo ridge mahukona hana ridge and offshore ni ihau form pointed cones of alkalic basalt and hawaiite the shield stage flat topped cones have steep 25 sides remarkably flat horizontal tops basal diameters of 1 3 km and heights 300 m the flat tops commonly have either a low mound or a deep crater in the center the rejuvenated stage flat topped cones have the same shape with steep sides and flat horizontal tops but are much larger with basal diameters up to 5 5 km and heights commonly greater than 200 m the flat tops have a central low mound shallow crater or levees that surrounded lava ponds as large as 1 km across most of the rejuvenated stage flat topped cones formed on slopes 10 and formed adjacent semicircular steps down the flank of ni ihau rather than circular structures all the flat topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades these and other submarine volcanic cones of similar size and shape apparently form as continuously overflowing submarine lava ponds a lava pond surrounded by a levee forms above a sea floor vent as lava continues to flow into the pond the lava flow surface rises and overflows the lowest point on the levee forming elongate pillow lava flows that simultaneously build the rim outward and upward but also dam and fill in the low point on the rim the process repeats at the new lowest point forming a circular structure with a flat horizontal top and steep pillowed margins there is a delicate balance between lava heat supply to the pond and cooling and thickening of the floating crust factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents moderate to high confining pressure at moderate to great ocean depth long lived steady eruption years to decades moderate effusion rates probably ca 0 1 km3 year and low but not necessarily flat slopes with higher effusion rates sheet flows flood the slope with lower effusion rates pillow mounds form hawaiian shield stage eruptions begin as fissure eruptions if the eruption is too brief it will not consolidate activity at a point and fissure fed flows will form a pond with irregular levees the pond will solidify between eruptive pulses if the eruption is not steady lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones as occurs during the post shield stage the steady effusion of lava on land constructs lava shields which are probably the subaerial analogs to submarine flat topped cones but formed under different cooling conditions

hawaiian xenolith populations magma supply rates and development of magma chambers hawaiian volcanoes pass through a sequence of four eruptive stages characterized by distinct lava types magma supply rates and xenolith populations magma supply rates are low in the earliest and two latest alkalic stages and high in the tholeiitic second stage magma storage reservoirs develop at shallow and intermediate depths as the magma supply rate increases during the earliest stage magma in these reservoirs solidifies as the supply rate declines during the alkalic third stage these magma storage reservoirs function as hydraulic filters and remove dense xenoliths that the ascending magma has entrained during the earliest and latest stages no magma storage zone exists and mantle xenoliths of lherzolite are carried to the surface in primitive alkalic lava during the tholeiitic second stage magma storage reservoirs develop and persist both at the base of the ocean crust and 3 7 km below the caldera only xenoliths of shallow origin are carried to the surface by differentiated lava during the alkalic third stage magma in the shallow subcaldera reservoir solidifies and crustal xenoliths including oceanic crustal rocks are carried to the surface in lava that fractionates in an intermediate depth reservoir worldwide xenolith populations in tholeiitic and alkalic lava may reflect the presence or absence of subvolcanic magma storage reservoirs

south arch volcanic field newly identified young lava flows on the sea floor south of the hawaiian ridge several young lava fields were imaged by gloria sidescan sonar along the hawaiian arch south of hawaii the largest 35 by 50 km across includes a central area characterized by high sonar backscatter and composed of several flow lobes radiating from a vent area reflection profiling and sea floor photography indicate that the central lobes are flat sheet flowsbounded by pillowed margins thin surface sediment and thin palagonite rinds on lava surfaces suggest ages of 1 10 ka vents are localized along the arch crest near bases of cretaceous seamounts two dredged flows are basanite and alkalic basalt broadly similar to rejuvenated stage and some pre shield alkalic lavas on the hawaiian ridge arch volcanism representsperipheral leakage of melt from the hawaiian hot spot over much larger areas than previously recognized

paleosecular variation of brunhes age lava flows from british columbia canada 1 brunhes age lava flows have been sampled for paleosecular variation studies from several volcanic fields of southern british columbia silverthrone garibaldi lake mt meager clearwater wells gray park and kelowna area a total of 52 lava flows were sampled and 7 to 10 samples were drilled at each site previous radiometric studies indicate that the ages of these lava flows range from 2 3 to 760 ka stepwise thermal demagnetization 14 to 21 steps was carried out for all the samples in each site and af demagnetization was performed on one sample per site forty five sites were selected based on rigorous criteria 95 5 to calculate a mean direction d 356 9 i 70 2 95 2 8 that is statistically indistinguishable from the direction of the dipole field at the area i 68 3 virtual geomagnetic poles vgps do not show the far sided effect and the angular standard deviation is 17 5 a value in agreement with the paleomagnetic field dispersion for that latitude these high quality results are expected to improve the time averaged field taf and secular variation models

evolution of alkalic lavas at haleakala volcano east maui hawaii the postshield and posterosional stages of haleakala volcano contain intercalated alkalic basalt and evolved alkalic lavas isotopic and incompatible element abundance ratios in the haleakala postshield basalts changed systematically with time providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources specifically a depleted i e low87sr 86sr and high143nd 144nd mantle component is more abundant in younger lavas however as magma production rates decreased during the postshield and posterosional stages basaltic melts in magma reservoirs cooled and fractionated leading to evolved residual melts such as hawaiite because primary basalt compositions changed with time the evolved haleakala lavas formed from a range of parental compositions however basalts and evolved lavas of similar age and isotopic ratios sr and nd have major and trace element contents that are consistent with a crystal fractionation model although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both haleakala and mauna kea volcanoes there are important differences between their lavas for example compositional differences between the hawaiite suites at haleakala and mauna kea indicate that on average the evolved lavas at haleakala formed at lower pressures also at haleakala basalts are intercalated with hawaiites whereas at mauna kea basalts and hawaiites are separated by a sharp boundary these differences probably reflect a higher magma supply rate to the haleakala volcano

new k ar ages and the geologic evidence against rejuvenated stage volcanism at haleakalā east maui a postshield stage volcano of the hawaiian island chain the postshield and previously inferred rejuvenated stage history of haleakal volcano is reevaluated on the basis of 52 new k ar ages 42 from the postshield kula volcanics and 10 from the overlying h na volcanics postshield extrusion was robust from 0 93 to 0 76 ma a period of low extrusion rate or volcanic quiescence occurred between 0 76 and 0 65 ma well within kula time a chemical change to increasingly alkalic lava occurred at this time as the volcano changed from broadly hawaiitic to basanitic in its eruptive products and robust extrusion resumed a slightly longer period of low extrusion rate or quiescence occurred after ca 0 4 ma but only trifling change in geochemical character is observed geochemically the h na volcanics unit chiefly basanitic overlaps greatly with the upper part of the kula volcanics there is a weak tendency to slightly more alkaline character among the h na volcanics the age of the kula h na boundary is ca 0 15 0 12 ma thus volcanic quiescence of only 0 03 m y separates the two formations much shorter than the previously known limit of 0 25 0 30 m y the brevity of this hiatus coupled with coincident vent loci and broadly similar geochemical characteristics for the h na and the upper part of the kula volcanics indicates that the h na volcanics unit comprises deposits of postshield stage volcanism that has waned substantially since ca 0 4 0 3 ma haleakal has not yet begun a classically defined rejuvenated stage our findings support recent numerical modeling of plume lithosphere interactions that predict that haleakal is near the end of its postshield growth

geochemistry of kauai volcanics and a mixing model for the origin of hawaiian alkali basalts a comprehensive model is developed to explain the major trace element and strontium and neodymium isotopic characteristics of alkali basalts from hawaii the model is similar to that of chen and frey 1983 in that it requires mixing of a small melt fraction of morb source material with another component to generate the alkalic suite of a particular hawaiian volcano it differs from the chen and frey model in that the other end member must be different from primitive mantle if it is to be consistent with both trace element and isotopic data alkali basalts and tholeiites from kauai analyzed in this study show a nearly complete transition in sr and nd isotopes there is a relatively well constrained array on a nd sr isotope correlation plot that can be explained by two component mixing of kauai tholeiite magma and a small amount of melt of east pacific rise source rock after corrections are made for fractional crystallization involving primarily clinopyroxene and olivine the sr and ba concentrations of kauai lavas plot along mixing curves defined by the above sources providing positive tests of the mixing hypothesis implications of this model are 1 the main source of hawaiian shield building tholeiites is a mixture of subducted crust primitive mantle and depleted asthenosphere that has been homogenized prior to melting 2 early alkalic volcanism as at loihi seamount will be characterized by greater isotopic heterogeneity than will late stage alkali basalt production and 3 there are two fundamentally distinct types of alkalic lavas erupted towards the end of magmatism at a given hawaiian volcano one represents smaller degrees of melting of the same source that generated shield building tholeiites kohala type the other derives from the mixed source discussed in this paper haleakala kauai type