multiple sources of quaternary tephra layers in the mariana trough abstract tephra layers in quaternary sediments stolper and newman 1994 model of binary mixing of source components with one endmember being the nmorb source of stolper and newman 1994 and the other one being a compositionally variable slab derived h2o rich component similar to the h2o component of stolper and newman 1994 this interpretation implies that the variation in k2o content in the arc derived glasses about a factor of 3 at 60 wt sio2 reflects a magmatic diversity that is inherent to the central mariana arc and is not necessarily reflective of temporal changes of the composition of arc magma sources

contrasting compositions of mariana trough fallout tephra and mariana island arc volcanics a fractional crystallization link discrete quaternary 400 ka tephra fallout layers mostly 1 cm thick within the siliceous oozes of the central mariana trough at 18 n are characterized by medium k to high k subalkalic volcanic glasses k2o 0 8 3 2 wt with high large ion lithophile elements lile high field strength elements hfse ratios and nb depletion ba la 35 ba zr 3 5 la nb 4 typical for convergent margin volcanic rocks compositional zoning within layers ranges from basaltic to dacitic sio2 48 71 wt mgo 0 7 6 5 wt all layers contain basaltic andesites the tephra layers are interpreted as single explosive eruptive events tapping chemically zoned reservoirs the sources being the mariana arc volcanoes mav due to their proximity 100 400 km and similar element ratios mav ba la 36 7 ba zr 3 5 0 9 the glasses investigated however contrast with the contemporaneous basaltic to dacitic lavas of the mav by being more enriched in tio2 1 2 wt mav 0 8 wt feo 10 wt mav 8 9 wt k2o 1 1 wt mav 0 8 wt and p2o5 0 4 wt mav 0 2 wt semi incompatible trace elements including rare earth elements ree of the basaltic andesitic and dacitic glasses match those of the dacitic mav lavas which became enriched by fractional crystallization moreover the glasses follow a tholeiitic trend of fractionation in contrast to mav transitional trends and have a characteristic p2o5 trend that reaches a maximum of 0 6 wt p2o5 at 57 wt sio2 whereas mav lavas increase linearly in p2o5 from 0 1 to 0 3 wt with increasing silica both explosive and effusive series are interpreted to have evolved in common magma reservoirs by convective fractionation similar parental magmas are suggested to have separated into coexisting si andesitic to dacitic and basaltic melts by in situ crystallization the differentiated melt is interstitial in an apatite saturated crystalline mush of plag px ox ol at the cooler chamber margins in contrast to the less differentiated basaltic to basaltic andesitic magmas which are not yet saturated in apatite and occupy the chamber interior reinjection of interstitial melt into the chamber interior and mixing with larger melt fractions of the interior liquid mixing ratios about 1 8 9 can explain the paradoxical behavior of apatite controlled p and mree variation in the basaltic andesite glasses and their mav dacite like fractionation patterns the process may also account for the exclusively tholeiitic trend of fractionation of the glass shard series but in situ crystallization alone cannot cause their absolute enrichment in semi incompatible elements the newly mixed melt is suggested to form the basaltic end member of the glass shard series however it must have become physically separated from the main mav magma body possibly by density driven convective fractionation in order to allow for further evolution of the contrasting geochemical paths as well as differentiation

petrology and geochemistry of lavas from the sumisu and torishima backarc rifts thirteen dredge hauls from the active sumisu and torishima rift grabens west of the izu bonin arc at about 30 31 n 140 e recovered a suite of tholeiitic basalts to sodic rhyolites volcanism occurs along tensional faults within and bounding the rift grabens and along the transfer zones between adjoining rift segments the sumisu and torishima rift lavas differ significantly from the lavas of the adjacent arc volcanic centers in having loweral2o3 na2o ba zr v ti andba ce and higher abundances of the rare earth elements the rift lavas also have characteristics of backarc basin basalts in that they are enriched in al2o3 and depleted in total iron and tio2 relative to mid ocean ridge basalt characteristics which are consistent with a higher water content in the source thus the model of a progressive change in backarc basin basalt composition from arc like to mid ocean ridge like as a function of evolution of the basin as has been suggested from many backarc regions is not generally applicable the comparison of the sumisu and torishima rift lavas with mariana backarc basin lavas indicates that backarc basin basalts differ in composition from one basin to another the comparison of these backarc basin suites with mid ocean ridge suites from similar axial depths indicates that the overall control over the spectrum of backarc basin basalt compositions may be different extents of melting of the mantle the sumisu and torishima rift lavas formed by a slightly higher extent of melting than the mariana backarc basin basalts a phenomenon which is related to the depth of the ridge segments furthermore the data suggest a systematically higher extent of melting in the arc lavas than in the backarc lavas for both of these arc backarc systems consistent with a greater flux of water beneath the arc than beneath the backarc region

mariana trough basalts mtb trace element and srnd isotopic evidence for mixing between morb like and arc like melts abstract mariana trough basalt mtb glasses from zones of of active seafloor volcanism have incompatible trace element compositions which are intermediate between normal morb and basaltic rocks from the active northern mariana island arc miab the chemical variation is observed in trace elemental abundances and ratios such as lil lil and lil hfs mtb glasses with high lil hfs and ba sm ratios and low k rb k ba and sm nd ratios have more enriched sr and nd isotopic compositions comparison of the sr nd isotopic compositions of mtb and miab suggests that the source region within the mantle wedge is heterogeneous the diverse trace element and isotopic compositions of mtb glasses both within and between dredge sites near 18 n imply small scale source heterogeneity correlation between sm nd and143nd 144nd of the mtb glasses is interpreted as due to recent binary mixing rather than closed system evolution of a common homogeneous source mixing of melts at or near the source region between a mantle component with long term lree and lil element depletion morb like and a relatively enriched component with lower integrated143nd 144nd arc like is suggested by trends of the mtb data on ratio ratio ratio element and element element plots

silicic volcanism at an off axis geothermal field in the mariana trough back arc basin an abyssal hill site with localized high heat flow on the 3 m y old west flank of the active ridge in the mariana trough has sets of oblique fault scarps mapped by multi beam transits and clusters of 50 to 1 000 m diameter domes examined with a deep tow survey some of the domes including small 20 to 30 m high rocky knobs encrusted with manganese oxide crop out through the sediment cover but despite heat flow evidence that they are sites of hydrothermal discharge these bedrock features are fundamentally different from the hydrothermal mounds found on some mid ocean ridges dredging one dome recovered a rhyodacite pumice that we believe erupted at a depth of 2 600 3 600 m within the basin the siliceous rock may be an extreme differentiate of a tholeiitic magma but a more likely origin is partial melting of the basin s sedimentary fill

origin of k2o sio2 trends in volcanoes of the mariana arc lavas from volcanoes in the mariana arc show a range of k 2 o sio 2 trends even though the volcanoes are all nearly the same distance above the benioff zone the preferred petrologic explanation for the k 2 o sio 2 variations invokes differences in the degree of partial melting required to produce lavas for each volcano from a relatively homogeneous source material the mariana arc volcanoes appear to evolve chemically in a manner similar to oceanic volcanoes e g hawaii with early and late alkalic stages dominated by an intermediate tholeiitic stage if volcanoes in other arcs also undergo this evolution many of the anomalies in k 2 o depth relations in modern arcs could be explained

petrology of the axial ridge of the mariana trough backarc spreading center abstract the axial ridge of the mariana trough backarc basin between 17 40 n and 18 30 n rises as much as 1 km above the floor of a 10 15 km wide rift valley physiographic segmentation with minor ridge offsets and overlaps coincides with a petrologic segmentation seen in trace element and isotope chemistry analyses of 239 glass and 40 aphyric basalt samples collected with alvin and by dredging show that the axial ridge is formed largely of olivine hypersthene normative tholeiitic basalt about half of these are enriched in both lil elements and volatiles but are depleted in hfs elements like other rocks found throughout much of the mariana trough the lil enrichments distinguish these rocks from n morb even though nd and sr isotope ratios indicate that much of the crust formed from a source similar to that for n morb in addition to lil enriched basalt there is lil depleted basalts even more closely resembling n morb in major and trace elements as well as sr nd and pb isotopes both basalt varieties have higher al and lower total fe than morb at equivalent mg level mg ranges from relatively primitive e g mg 65 70 to more highly fractionated e g mg 45 50 highest parts of the axial ridge are capped by pinnacles with elongated pillows of basaltic andesite e g 52 56 sio 2 these are due to extreme fractional crystallization of basalts forming the axial ridge active hydrothermal vents with chimneys and mats of opaline silica barite sphalerite and lesser amounts of pyrite chalcopyrite and galena formed near these silicic rocks the vents are surrounded by distinctive vent animals polychaete worms crabs and barnacles isotope data indicate that the mariana trough crust was derived from a heterogeneous source including mantle resembling the morb source and an arc source component the latter was depleted in hfs elements in previous melting events and later modified by addition of h 2 o and lil elements

nd and sr isotopic compositions of lavas from the northern mariana and southern volcano arcs implications for the origin of island arc melts nd and sr isotopic data are reported for lavas from 23 submarine and 3 subaerial volcanoes in the northern mariana and southern volcano arcs values of end range from 2 4 to 9 5 whereas 87sr 86sr ranges from 0 70319 to 0 70392 these vary systematically between and sometimes within arc segments the nd and sr isotopic compositions fall in the field of ocean island basalt oib and extend along the mantle array lavas from the volcano arc mariana central island province and the southern part of the northern seamount province have end to 10 and 87sr 86sr 0 7032 to 0 7039 these are often slightly displaced toward higher 87sr 86sr at similar end in contrast those lavas from the northern part of the mariana northern seamount province as far north as iwo jima show oib isotopic characteristics with end and 87sr 86sr 0 7035 to 0 7039 plots of 87sr 86sr and end versus ba la and la yb n support a model in which melts from the mariana and volcano arcs are derived by mixing of oib type mantle or melts therefrom and a metasomatized morb type mantle or melts therefrom an alternate interpretation is that anomalous trends on the plots of nd and sr isotopic composition versus incompatible element ratios found in some s nsp lavas suggest that the addition of a sedimentary component may be locally superimposed on the two component mixing of mantle end members

physical volcanology of the submarine mariana and volcano arcs narrow beam maps selected dredge samplings and surveys of the mariana and volcano arcs identify 42 submarine volcanos observed activity and sample characteristics indicate 22 of these to be active or dormant edifices in the volcano arc are larger than most of the mariana arc edifices more irregularly shaped with numerous subsidiary cones and regularly spaced at 50 70 km volcanos in the mariana arc tend to be simple cones sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110 counterclockwise from that trend suggesting a strong fault control on the distribution of arc magmas volcanos in the mariana arc are generally developed west of the frontal arc ridge on rifted frontal arc crust or new back arc basin crust volcanos in the central mariana arc are usually subaerial large 500 km3 and spaced about 50 70 km apart those in the northern and southern marianas are largely submarine closer together and generally less than 500 km3 in volume there is a shoaling of the arc basement around iwo jima accompanied by the appearance of incompatible element enriched lavas with alkalic affinities the larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos if the magma supply estimated at 10 20 km3 km of arc per million years at 18 n has been relatively constant along the mariana arc we can infer a possible evolutionary sequence for arc volcanos from small irregularly spaced edifices to large over 1000 km3 edifices spaced at 50 70 km the volcano distribution and basal depths are consistent with the hypothesis of back arc propagation into the volcano arc

shoshonitic magmas in nascent arcs new evidence from submarine volcanoes in the northern marianas volcanoes in the northern mariana arc between uracas lat 20 n and minami iwo jima 24 n are very active yet entirely submarine in contrast to the predominantly low k basaltic magmas of the central mariana arc the northern mariana arc is dominated by more siliceous melts in the south and by shoshonites in the north the northern arc melts have enrichments in ba 800 ppm rb 70 ppm sr 1000 ppm ce 50 ppm and ce yb n 24 which increase to the north as far as iwo jima lavas from volcanoes north of iwo jima lack these enrichments and are indistinguishable from those of the central maranas the shoshonites are unusual in occurring along the magmatic front of a primitive intra oceanic arc we hypothesize that they represent the reconstruction of a magmatic arc following melting of enriched mantle due to the propagation of the mariana trough spreading center northward through the volcano arc shoshonites thus may characterize the initial stages of arc construction after an episode of back arc rifting and need not be restricted to the mature stages of arc evolution this situation contrasts with subduction zone initiation where first melts may be boninites or low k tholeiites these differing initial melts converge toward tholeiitic and calc alkaline compositions as arcs evolve

submarine arc volcanism in the southern mariana arc as an ophiolite analogue supra subduction zone ssz ophiolites are being recognized more and more frequently in the geologic record submarine volcanism in intra oceanic arcs is one tectonic setting in which such ophiolites might form the southern seamount province of the mariana arc represents the best documented example of an active intra oceanic submarine arc which can serve as a modern analogue for some ssz ophiolites the southern seamount province comprises nine submarine volcanoes at least seven of which are active or dormant these edifices have erupted plagioclase clinopyroxene olivine phyric basalts andesites dacites and dacitic pumices these lavas are similar to those of the subaerial edifices in the central marianas sio2 49 70 tio2 0 4 1 3 mgo 1 2 5 9 ba 140 450 ppm 87sr 86sr 0 70346 0 70354 but are somewhat more enriched in the light rare earth elements the geochemical characteristics of subaerial mariana arc lavas are preserved in the submarine lavas and the ssp lavas are slightly enriched in incompatible elements relative to those of the subaerial edifices we are ignorant of the composition of any lavas erupted between the principal edifices which may be expected to dominate the volcanic section of ssz ophiolites nevertheless the characteristic high ratio lil hfsc low ti ni and cr and diagnostic mineral compositions calcic plagioclase relatively fe rich olivine are likely to be preserved in unaltered submarine arc lavas the mariana active arc is built on back arc or thinned frontal arc crust and any ophiolite formed from such an arc is likely to exhibit complex intrusive and eruptive relationships of plutonic hypabyssal and volcanic rocks of back arc frontal arc and active arc origin such complicated intrusive relationships if preserved in structurally disrupted ophiolites will make it difficult to reconstruct the geologic history of an ssz ophiolite

controls on space time distribution of soft sediment deformation structures applying palaeomagnetic dating to approach the apparent recurrence period of paleoseisms at the concud fault eastern spain abstract this work describes soft sediment deformation structures clastic dykes load structures diapirs slumps nodulizations or mudcracks identified in three sections concud ramblillas and masada cociero in the iberian range spain these sections were logged from boreholes and outcrops in upper pliocene lower pleistocene deposits of the teruel concud residual basin close to de concud normal fault timing of the succession and hence of seismic and non seismic ssdss covering a time span between 3 6 and 1 9 ma has been constrained from previous biostratigraphic and magnetostratigraphic information then substantially refined from a new magnetostratigraphic study at masada cociero profile non seismic ssdss are relatively well correlated between sections while seismic ones are poorly correlated except for several clusters of structures between 29 and 35 seismic deformed levels have been computed for the overall stratigraphic succession factors controlling the lateral and vertical distribution of ssdss are their seismic or non seismic origin the distance to the seismogenic source concud fault the sedimentary facies involved in deformation and the observation conditions borehole core vs natural outcrop in the overall stratigraphic section seismites show an apparent recurrence period of 56 to 108 ka clustering of seismic ssdss levels within a 91 ka long interval records a period of high paleoseismic activity with an apparent recurrence time of 4 8 to 6 1 ka associated with increasing sedimentation rate and fault activity such activity pattern of the concud fault for the late pliocene early pliocene with alternating periods of faster and slower slip is similar to that for the most recent quaternary last ca 74 ka bp concerning the research methods time occurrence patterns recognized for peaks of paleoseismic activity from ssdss in boreholes are similar to those inferred from primary evidence in trenches consequently apparent recurrence periods calculated from ssds inventories collected in borehole logs close to seismogenic faults are comparable to actual recurrence times of large paleoearthquakes

petrology chemistry and isotopic composition of submarine volcanoes in the southern mariana arc four submarine volcanoes in the southern and central mariana arc were dredge sampled the recovered lavas were analyzed for major and trace element abundances including rare earth elements ree mineral compositions and sr and nd isotopic ratios the samples exhibit a great range of chemical and mineral compositions relative to subaerial exposures in the central arc zealandia bank produces high magnesium basalts mgo 8 9 wt enriched in ni and with forsteritic olivine as much as fo 88 this lava composition is probably parental to more fractionated basalts and basaltic andesites exposed on the adjacent islands of guguan and sarigan the southernmost seamounts in the mariana arc south of lat 15 n produce dacitic pumice a composition that is not common in the active arc ree data preclude their derivation from a basaltic parent by fractional crystallization they may be derived by direct partial mantle melting under water saturated conditions at low pressure nd and sr isotopic data 143 nd 144 nd 0 5130 0 5131 87 sr 86 sr 0 7032 0 7035 indicate a mantle source for the mariana arc lavas similar in isotopic characteristics to sources for ocean islands significant contributions to these arc magmas from subducted oceanic crust or sediment are not evident a spatial distribution in magmatic characteristics coincides with a structural break in the arc near 16 n south of this submarine seamounts produce incompatible element enriched lavas with relatively high 87 sr 86 sr 0 7034 0 7035 these lavas were produced by a relatively low degree of partial melting 5 15 of a shallow spinel bearing mantle to the north zealandia bank and the central arc islands are characterized by lower incompatible element abundances and lower 87 sr 86 sr 0 7032 0 7033 these lavas were produced by higher degrees of partial melting 15 25 of a deeper garnet bearing mantle

fallout of pyroclastic debris from submarine volcanic eruptions volcanic fallout deposits on land being widespread and accessible for study have received much attention and have revealed a great deal about subaerial eruption mechanisms in contrast virtually nothing is known about equivalent deposits produced by submarine volcanoes despite the probable abundance of such material in today s oceans and in accreted volcanic arc terrains many submarine deposits may form by the fallout of debris to the sea floor downcurrent from the umbrella region of submarine eruption columns experiments on water saturated pumice and pieces of rock lithics show that particles settling to the sea floor at terminal velocities of 10 to 50 centimeters per second will display conspicuous bimodality of particle diameters pieces of pumice may be five to ten times as large as codeposited lithic fragments similar material erupted into the air and deposited on land displays less well developed bimodality pumice diameters are generally two to three times as large as associated lithics submarine fallout deposits are therefore distinctive and may be used to indicate a subaqueous origin for some of the great thicknesses of nonfossiliferous volcanic debris contained in ancient volcanic terrains worldwide whose environment of deposition has been uncertain

depths and water content of magma chambers in the aleutian and mariana island arcs on pseudoternary diagrams locations of experimentally produced melt compositions that coexist with plagioclase and anhydrous mafic minerals vary as functions of pressure a geobarometer and water content a geohygrometer projection of rock compositions onto diagrams allows constraints to be placed on magma chamber depths and meltwater content application of the experimentally calibrated geobarometer and gehygrometer to aleutian and mariana rock suites suggests aleutian magma chambers to be at depths of 7 to 17 km pressures from 2 to 5 kbar and meltwater content of andesites near 2 wt mariana magma chambers are at shallow depths pressure less than 2 kbar and their water content is less than 2 wt probably near 1 wt magma chamber depths can be interpreted to be controlled by density differences between crystallizing magmas and crust which in turn may be controlled by crustal thermal structure differences in water content are correlated with amounts of sediment incorporated into basaltic magmas formed near the subducting oceanic slab

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

seasonal climate signals 1990 2008 in a modern soreq cave stalagmite as revealed by high resolution geochemical analysis article high resolution isotopic and geochemical analyses in a modern 1990 2008 soreq cave stalagmite are com pared to instrumental records of rainfall and dripwater from the cave with the aim of determining how seasonal resolutionclimateinformationistransmittedtospeleothemgeochemistry insitu micron scaleanalysis of oxygen isotope ratios 18 o and trace elements by ion microprobe in combination with a continuous linear traverse of trace element concentrations by laser ablation icp ms la icp ms allow the definition of geochem ical pathways within the cave fluorescent banding imaged by confocal laser fluorescent microscopy clfm as well as

cross arc geochemical variations in the kurile arc as a function of slab depth lavas from transects across the kurile islands arc showed geochemical variations related to changes in the compositions of fluids derived from the subducting slab enrichment factors for boron cesium arsenic and antimony all elements with strong affinities for water decreased across the arc this decrease is presumably related to losses of water rich fluids during the dehydration of the subducting plate enrichments of potassium barium beryllium 10 and the light rare earth elements remained constant these species may move in silica rich fluids liberated from the slab at greater depths

esmeralda bank geochemistry of an active submarine volcano in the mariana island arc esmeralda bank is the southernmost active volcano in the izu volcano mariana arc this submarine volcano is one of the most active vents in the western pacific it has a total volume of about 27 km3 rising to within 30 m of sea level two dredge hauls from esmeralda recovered fresh nearly aphyric vesicular basalts and basaltic andesites and minor basaltic vitrophyre these samples reflect uniform yet unusual major and trace element chemistries mean abundances of tio2 1 3 and feo 12 6 are higher and cao 9 2 and al2o3 15 1 are lower than rocks of similar silica content from other active mariana arc volcanoes mean incompatible element ratios k rb 488 and k ba 29 of esmeralda rocks are indistinguishable from those of other mariana arc volcanoes on a ti zr plot esmeralda samples plot in the field of oceanic basalts while other mariana arc volcanic rocks plot in the field for island arcs

shoshonitic volcanism in the northern mariana arc 1 mineralogic and major and trace element characteristics petrographic and geochemical characteristics of samples dredged from 23 submarine volcanic edifices in the northern mariana and southern volcano arcs define two distinct rock series the central and northern mariana arc to 23 n and the northern volcano arc north of iwo jima are characterized by plagioclase clinopyroxene orthopyroxene titantomagnetite bearing subalkaline rocks including both low k and medium k series the northern mariana arc and southern volcano arc from 23 n to iwo jima are erupting rocks of a shoshonitic series with phenocrysts of plagioclase clinopyroxene olivine and biotite these rocks are less saturated than those of the subalkaline provinces and are substantially enriched in ba 400 900 ppm sr 600 1000 ppm k2o 1 4 5 and k2o na2o 0 4 1 2 relative to the subalkaline lavas ba y and ba zr increase by a factor of 3 to 4 in the shoshonitic rocks but k rb ba sr and k ba are relatively constant throughout the arc there is no relationship between degree of enrichment and volcano volume or degree of fractionation seamounts and islands within each province have the same range of compositions much of the intra edifice variation in lava composition can be modeled by 10 70 crystallization of plagioclase and clinopyroxene with smaller amounts of olivine orthopyroxene and titanomagnetite or by accumulation of 10 40 phenocrysts dominantly plagioclase the differences between parental lavas in the alkalic and subalkalic provinces require melting of two distinct mantle sources the occurrence of this enriched mantle in the northern marianas may be a consequence of the propagation of the mariana trough spreading center into the volcano arc

the eruption of mount pagan volcano mariana islands 15 may 1981 a major explosive eruption occurred 15 may 1981 at mount pagan volcano the larger of two historic eruptive centers on pagan island mariana islands the eruption was preceded by increased numbers of locally felt earthquakes beginning in late march or early april and by new ground cracks new sublimates and increased gas emissions a swarm of felt earthquakes began at 0745h local time uct 10 hours 15 may and at 0915 h closely following a loud sonic boom a strong plinian column issued from the volcano the high altitude ash cloud at least 13 5 km travelled south southeast but ash and scoria deposits were thickest 2 m in the nw sector of the island because of the prevailing low altitude southeasterly winds the early activity of 15 may probably involved magmatic eruption along a fissure system oriented about n10 e however the eruption became hydromagmatic possibly within minutes and was largely restricted to three long lived vents the northernmost of these built a substantial new scoria ash cinder cone flows and air fall deposits consisting almost entirely of juvenile material exceeded 105 106 m3 in volume 75 106 m3 of magma on land and at least 70 100 606 m3 at sea an unknown volume was carried away by stratospheric winds lithic blocks and juvenile bombs as large as 1 m in diameter were thrown more than 2 km from the summit and evidence for base surge was observed in restricted corridors as low as 200 m elevation on the north and south slopes of the volcano neither of these events resulted in serious injuries to the 54 residents of the island nor did the eruption produce serious chemical hazards in their water supply weak eruptions occurred during the ensuing month and some of these were monitored by ground observations seismic monitoring and deformation studies precursory seismicity and possibly deformation occurred with some of the observed eruptions more vigorous eruptions were reported by visiting residents in late 1981 and early 1982 but these were of lesser magnitude than the 15 may 1981 event the 15 may lava is predominantly aa and ranges from 3 to 30 m in thickness in composition it is a high alumina basalt with small 1 mm long phenocrysts of plagioclase and clinopyroxene 7 that is more or less typical of basalt of the northern marianas volcanoes it contains slightly more sio2 52 k2o tio2 and less al2o3 and cao than does the basalt of the last eruptive event of mount pagan volcano in 1925 gas analyses indicate that a large portion of air was introduced into the vent system through the porous volcanic edifice and that the carbon gases were not in equilibrium with the magma or each other

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

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