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

effective tension shear relationships in extensional fissure swarms axial rift zone of northeastern iceland abstract the geometry of fracture systems in selected areas of the active krafla fissure swarm mid atlantic ridge northeastern iceland is analysed based on geodetic analysis of the present day topography at the top of holocene basaltic lava flows which fill the axial rift zone the deformation of this initially horizontal surface can be reconstructed extensional deformation is localised at all scales and block tilting though present remains minor using simple models of the surface expression of normal faults the geometrical characteristics of the topographic features related to active deformation during tectonic volcanic events are quantitatively analysed at crustal depths of about 1 km normal faults are present and have an average 70 dip comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone at palaeodepths of 1 2 km indicates that this dip determination is valid comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin newly formed brittle crust of the rift zone in the axial oceanic rift zone of ne iceland the extensional deformation in the upper crust is dominated by horizontal tension and shear of normal sense their relative importance depending on depth absolute tension dominates in the uppermost several hundred metres of the crust resulting in the development of fissure swarms effective tension plays an important role at a deeper level 2 5 km because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections at crustal depths of about 1 km normal shear prevails along fault planes which dip 60 75 this importance of normal shear at moderate depth between upper and lower crustal levels where tension prevails is pointed out within the extensional context of rifting these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure

tectonic history of sumba island indonesia since the late cretaceous and its rapid escape into the forearc in the miocene abstract the island of sumba lies within the sunda banda forearc and contains volcanic plutonic and volcaniclastic rocks that record a history of arc volcanism extending from late cretaceous 80 ma or earlier to oligocene 31 ma time during the late cretaceous sumba formed part of a great indonesian volcanic arc system near southeastern eurasia between late cretaceous and early miocene time sumba remained part of that arc system which ceased to be volcanically active by 31 ma during the late miocene 18 ma sumba as part of the relict arc system was situated near the present site of alor and wetar at 16 ma sumba was torn from the relict arc and began to move at a velocity of 50 mm yr in a west south westerly direction volcanism along the modern banda arc soon followed by 7 ma sumba finally came to rest after having moved 450 km into the forearc we interpret the rapid motion of sumba into the forearc as a discrete block to have been a response to the collision between australia and indonesia in the area of timor the savu basin possibly originated as an arc crestal rift which then opened behind sumba as the latter moved westward over the past 16 my more than 3 km of sediment have been deposited in the savu basin by erosion from the surrounding islands sumba stopped moving to the west south west at 7 ma after reaching approximately its present location since then the island has ceased to be a discrete tectonic fragment and has behaved as an integral part of the forearc during the past 7 my compressional tectonics within the forearc has led to elevation of the rocks exposed on sumba by as much as 4 km

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

exploration of soils developing on volcanic materials on the island of milos greece abstract the island of milos greece part of the south aegean volcanic arc with a typical mediterranean climate is covered with volcanic deposits of different ages the objective of this study was to investigate the physicochemical and mineralogical properties of the soils developing on these volcanic deposits and their classification samples were taken from seven locations of soil on different parent material and of different ages there were substantial differences in their particle size distribution with sand ranging from 19 to 92 silt from 3 5 to 50 and clay from 5 to 46 organic matter content was low 3 equivalents of 1 4 to 24 6 were found and a calcic horizon identified the cation exchange capacity cec and specific surface area ssa varied between profiles ranging from 3 cmol kg 1 to 47 cmol kg 1 and 30 m 2 g 1 to 380 m 2 g 1 respectively the soils exhibited high base saturation the amounts of al fe and si extracted with ammonium oxalate l o fe o and si o were particularly low d was greater than fe o sharing the dominance of crystalline fe oxides al and si extracted with hot 0 5 m naoh al 2 3naoh and si 2naoh and with iron c 6 h 4 na 2 o 8 s 2 al 2 3 and si 2 si 2naoh and si 2 were particularly high mean values 3 4 and 4 5 respectively showing that amorphous silica was present the clay fraction of the soil was dominated by the presence of 2 1 vermiculite and smectite and 1 1 kaolinite clay silicates al o 1 2fe o was low

tholeiitic and calc alkaline magma series at adatara volcano northeast japan 1 geochemical constraints on their origin abstract low alkali island arc by jakes and gill 1970 or low k by gill 1978 tholeiitic rocks are closely associated with calc alkaline rocks in the outer volcanic zone called the nasu volcanic zone of northeast japan major element trace element ni cr co sc v zr nb zn rb cs ba sr ce and y and sr isotope compositions are presented for 9 low alkali tholeiitic and 16 calc alkaline samples of adatara volcano belonging to the nasu volcanic zone in the trace elements vs sio 2 diagram the tholeiitic series and calc alkaline series show different trends to each other for co sc v zr rb sc ba and sr as expected from k 2 o contents the calc alkaline samples are enriched in incompatible elements such as rb cs ba ce and zr compared with the tholeiitic samples on the other hand contents of nb in the calc alkaline samples are somewhat lower than those in the tholeiitic samples plots of ni and cr contents vs sio 2 for the tholeiitic samples come near smooth trend lines whereas those for the calc alkaline samples are scattered sr isotopic ratios in the tholeiitic series lie within the range between 0 7056 and 0 7058 and between 0 7048 and 0 7052 in the calc alkaline series smooth major and trace element compositional trends in the tholeiitic series are successfully modelled by simple fractional crystallization of plagioclase hypersthene augite titanomagnetite and minor olivine similarly crystal fractionation models can closely reproduce major and most of the trace element variations in the calc alkaline series but failed to account for irregular variations of ni and cr the scattering of ni and cr probably reflects simultaneous crystal fractionation and subordinate magma mixing in the subvolcanic chamber the inferred fractionated phases for the calc alkaline series are richer in augite and titanomagnetite and slightly poorer in plagioclase than those for the tholeiitic series the tholeiitic magma and calc alkaline magma are not derived from a common parental tholeiitic magma and the calc alkaline parental magma is richer in sio 2 and incompatible elements in particular rb and zr poorer in radiogenic sr and possibly more oxidized than the tholeiitic parental magma

evolution of vertical faults at an extensional plate boundary southwest iceland vertical faults having both opening and vertical displacements are common in southwest iceland and hypotheses vary regarding whether they propagated to the surface from below or from the surface downward we address this issue through a study of vertical faults and associated surface fracture zones in regions of both oblique and normal spreading in southwest iceland individual fracture segments are commonly rotated out of the general trend of the fracture zone suggesting oblique motion along subsurface normal faults these faults commonly breach the upper hinge lines of narrow monoclinal folds that flank many fault traces on the hanging wall side based on these field observations and the results of numerical models we propose that 60 758 dipping normal faults in the subsurface propagated to the surface from below vertical fractures formed at the upper tips of the faults at depths of between 250 and 500 m 25 50 of the fault length in response to stress concentrations along the tip line model results indicate that narrow monoclinal folds develop at the surface above these vertical fractures which subsequently breach the monoclines along the upper hinge line forming vertical fault scarps and open fissures at the surface if vertical fractures utilize pre existing cooling joints in basalt to connect directly to the surface the hanging wall is able to pull apart from the footwall without the development of a surface monocline along the fault trace q 2003 elsevier ltd all rights reserved

influence of an inhomogeneous stress field and fault zone thickness on the displacements and stresses induced by normal faulting we used the finite element method fem to simulate the temporal evolution of an inhomogeneous stress field with the aim of investigating the displacement and stress fields induced by slip upon a normal fault zone with a given thickness the modeling results reveal 1 the site of maximum slip is along the lower part of the fault not near the surface and the locations upon the fault of maximum shear stress drop and maximum slip do not coincide 2 the maximum horizontal and vertical displacements at the ground surface are located at some distance from the fault trace and 3 slip of the fault may result in two failure regions near the ground surface one close to the surface trace of the fault and the other within the hanging wall at some distance from the fault it is in the regions that a succession of normal faults is typically observed in geological examples these results differ from those obtained using seismic dislocation theory in semi infinite space where the effects of the inhomogeneous stress field and thickness of the fault zone are not considered

igneous garnet and amphibole fractionation in the roots of island arcs experimental constraints on andesitic liquids to evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs a series of experiments were performed on a synthetic andesite at conditions ranging from 0 8 to 1 2 gpa 800 1 000 c and variable h2o contents at water undersaturated conditions and fo2 established around qfm garnet has a wide stability field at 1 2 gpa garnet amphibole are the high temperature liquidus phases followed by plagioclase at lower temperature clinopyroxene reaches its maximal stability at h2o contents 9 wt at 950 c and is replaced by amphibole at lower temperature the slopes of the plagioclase in boundaries are moderately negative in text t text x text h 2 text o space at 0 8 gpa garnet is stable at magmatic h2o contents exceeding 8 wt and is replaced by spinel at decreasing dissolved h2o the liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1 2 gpa series but show substantial enrichment in feo mgo for the 0 8 gpa series related to the contrasting roles of garnet and amphibole in fractionating fe mg in derivative liquids our experiments indicate that the stability of igneous garnet increases with increasing dissolved h2o in silicate liquids and is thus likely to affect trace element compositions of h2o rich derivative arc volcanic rocks by fractionation garnet controlled trace element ratios cannot be used as a proxy for slab melting or dehydration melting in the deep arc garnet fractionation either in the deep crust via formation of garnet gabbros or in the upper mantle via formation of garnet pyroxenites remains an important alternative despite the rare occurrence of magmatic garnet in volcanic rocks

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

tholeiitic aleutian arc plutonism the finger bay pluton adak alaska the earliest representatives in the sequence of tertiary to recent magmatic rocks on adak island in the central aleutians are the finger bay volcanics and associated small shallow level gabbroic intrusives the tholeiitic finger bay pluton gabbro to quartz monzodiorite is among the least altered representatives of this earliest period the field relations mineralogy and geochemistry of the finger bay pluton contrast with the more recent calcalkaline plutons in the arc e g on adak hidden bay pluton 33 m y and on kagalaska kagalaska pluton 15 m y compared with the hidden bay pluton the finger bay pluton is smaller 8 km2 versus 100 km2 has a greater proportion of gabbro 84 versus 5 has a somewhat different mineralogy and has higher whole rock and mafic mineral feo mgo ratios higher k2o and higher concentrations of incompatable elements in rocks of equivalent sio2 particularly for the more siliceous units magmatic amphibole occurs only in the most siliceous units in the finger bay pluton whereas it is common in the calc alkaline plutons except for the size these differences parallel those between tholeiitic and calc alkaline volcanic rocks from the active aleutian volcanic centers textural miner alogical and trace element analyses indicate that mixing of magmas and gabbros generated some of the rocks of intermediate monzodiorite composition in the finger bay pluton most of the differences in magmatic trends between the calc alkaline and tholeiitic plutons are attributed to the physical conditions of magmatic evolution in the crust rather than to differences in initial magmatic types this conclusion rests on the similarity of fractionation independent isotope ratios and trace element ratios in aleutian magmas of all ages in particular compared to magmas of the north pacific ocean basin morb oceanic islands aleutian magmas show excess enrichment of ba k rb and u relative to ree and high 207pb 204pb ratios for a given 206pb 204pb ratio recognition of tholeiitic series plutons in the arc provides direct evidence for magmatic conditions accompanying fractionation and serves to emphasize the diversity of magmatic trends that are found over time and space at convergent plate margins

magmatic evolution of semisopochnoi island alaska trace element and isotopic constraints semisopochnoi has erupted a tholeiitic suite of moderately evolved lavas sio 2 50 65 zr 64 250 ppm la 10 60x chondrites whose bulk rock chemistry and phenocryst mineralogy are generally compatible with an important role for fractional crystallization in controlling the overall evolution of the suite trace element data however are not consistent with crystal fractionation operating alone instead a variety of plots involving incompatible elements or ratios among them demonstrate that simple mixing between basaltic and silicic components can reproduce the essential aspects of the trace element variations and must have been operative as well we are unable to distinguish between a silicic end member represented by residual dacitic liquid like one of the analyzed lavas therefore implying magma mixing and one consisting of pre existing silicic crustal rocks therefore implying assimilation variations in sr and nd isotopic compositions are very limited 143 nd 144 nd 0 51225 0

co existing calcic amphiboles in calc alkaline andesites possible evidence of a zoned magma chamber abstract hornblende biotite andesites erupted from mount price and clinker peak volcanoes southwestern british columbia contain two texturally and compositionally distinct calcic amphiboles pargasitic hornblende xenocrysts and magnesio hornblende microphenocrysts disequilibrium relationships exhibited by these amphiboles and associated minerals suggest that the magnesio hornblendes precipitated under chemical and thermal conditions that were intermediate between those under which pargasitic hornblende and biotite respectively crystallized experimental studies of crystallization in double diffusive systems chen and turner 1980 turner 1980 mcbirney 1980 suggest that these varied magmatic environments can be explained as a consequence of progressive crystallization within a zoned magma chamber although gravitational settling may have played a role the observed mineral assemblages probably developed by convective mixing of crystals precipitated at the cooling margins with those crystallized in the interior of the compositionally stratified magma column

aleutian terranes from nd isotopes nd isotope ratios substantiate the identification of oceanic crustal terranes within the continental crustal basement of the aleutian island arc the oceanic terranes are exposed in the westernmost aleutians but to the east they are completely buried by isotopically distinct arc volcanic rocks analogous oceanic terranes may be important components of the terrane collages that comprise the continents

investigation of soils developed on volcanic materials in nisyros island greece abstract soils that are forming on volcanic parent materials have unique physical and chemical properties and in most cases on wet and humid climates are classified as andisols the main purpose of this study is to examine if the soils that are forming on volcanic materials under a dry mediterranean climate in nisyros island greece meet the requirements to be classified as andisols soils from seven sites were sampled and examined for their main physico chemical properties and selective dissolution analysis dithionite citrate bicarbonate dcb extractable al and fe al d fe d acid ammonium oxalate extractable al fe and si al o fe o and si o and sodium pyrophosphate extractable al and fe al p fe p were measured in addition al and si were determined after reaction with hot 0 5 m naoh al naoh and si naoh and with tiron c 6 h 4 na 2 o 8 s 2 al t and si t p retention was also measured the soils are characterised by coarse texture low organic matter content low values of cation exchange capacity cec and high ph values values of si o al o and fe o are less than 0 022 0 09 and 0 35 respectively highlighting the lack of noncrystalline components the ratio fe d fe o 100 fe d is quite high expressing the degree of crystallisation of free iron oxides for all samples tested values of the al o 1 2fe o index are extremely low naoh and si t arising 2 76 and 2 18 respectively indicate the presence of silica in amorphous forms p retention values are very low

aleutian tholeiitic and calc alkaline magma series i the mafic phenocrysts diagnostic mafic silicate assemblages in a continuous spectrum of aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the aleutian arc crust tectonic segmentation of the arc exerts a primary control on the variable mixing fractional crystallization and possible assimilation undergone by the magmas end members of the continuum are termed calc alkaline ca and tholeiitic th ca volcanic rocks e g buldir and moffett volcanoes have low feo mgo ratios and contain compositionally diverse phenocryst populations indicating magma mixing their ni and cr rich magnesian olivine and clinopyroxene come from mantle derived mafic olivine basalts that have mixed with more fractionated magmas at mid to lower crustal levels immediately preceding eruption high al amphibole is associated with the mafic end member in contrast th lavas e g okmok and westdahl volcanoes have high feo mgo ratios and contain little evidence for mixing evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma these lavas contain groundmass olivine fo 40 50 and lack ca poor pyroxene aleutian volcanic rocks with intermediate feo mgo ratios are termed transitional tholeiitic tth and calc alkaline tca tca magmas are common e g moffett adagdak great sitkin and kasatochi volcanoes and have resulted from mixing of high al basalt with more evolved magmas they contain amphibole high and low al or orthopyroxene or both and are similar to the japanese hypersthene series tth magmas e g okmok and westdahl contain orthopyroxene or pigeonite or both and show some indication of upper crustal mixing they are mineralogically similar to the japanese pigeonite series high al basalt lacks mg rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite the low pressure mineral assemblage of high al basalt results from crystallization at higher crustal levels

andesites from northeastern kanaga island aleutians kanaga island is located in the central aleutian island arc northeastern kanaga is a currently active late tertiary to recent calc alkaline volcanic complex basaltic andesite to andesite lavas record three episodes series of volcanic activity series i and series ii lavas are all andesite while series iii lavas are basaltic andesite to andesite four series ii andesites contain abundant quenched magmatic inclusions ranging in composition from high mgo low alumina basalt to low mgo highalumina basalt the spectrum of lava compositions is due primarily to fractional crystallization of a parental low mgo high alumina basalt but with variable degrees of crustal contamination and magma mixing the earliest series i lavas represent mixing between high alumina basalt and silicic andesite with maximum sio2 contents of 65 67 wt later series i and all series ii lavas are due to mixing of andesite magmas of similar composition the maximum sio2 content of the pre mixed andesites magmas is estimated at 60 63 wt the youngest lavas series iii are all non mixed and have maximum estimated sio2 contents of 59 wt the earliest series i lavas contain a significant crustal component while all later lavas do not it is concluded that the maximum sio2 contents of silicic magmas the contribution of crustal material to silicic magma generation and the role of magma mixing all decrease with time furthermore silicic magmas generated by fractional crystallization at this volcanic center have a maximum sio2 content of 63 wt all of these features have also been documented at the central aleutian cold bay volcanic center brophy 1987 based on data from these two centers a model of aleutian calc alkaline magma chamber development is proposed the main features are 1 a single low pressure magma chamber is continuously supplied by primitive low alumina basalt 2 non primary high alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase 3 sidewall crystallization accompanied by crustal melting produces buoyant silicic 63 wt sio2 liquids that pond at the top of the chamber and 4 continued sidewall crystallization now isolated from the chamber wall produces silicic liquids with 63 wt sio2 that increase the thickness and lowers the overall sio2 content of the upper silicic zone it is suggested that the maximum sio2 content of 63 imposed on fractionation generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal liquid mush along the chamber wall

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

development application and evaluation of a methodology to estimate distributed slip on fractures due to future earthquakes for nuclear waste repository performance assessment the design and licensing of underground repositories to safely store high level nuclear waste requires the conservative estimation of the risks that earth quakes might pose in order to meet objectives set forth by the swedish government for the safe disposal of spent fuel the swedish program has postulated a scenario in which the energy released by future earthquakes over the next 100 000 yr may pro duce slip on fractures intersecting canisters and they have begun a probabilistic performance assessment of this scenario conventional methods for the assessment of seismic impacts to population centers or to relatively short lived surface structures may not be useful for this purpose moreover there are difficulties in using proba bilistic functions calibrated from field data on distributed slip in the swedish case such as has been proposed for the u s program to overcome these inadequacies a new methodology has been developed and applied to three sites in sweden that serve as laboratories for the design and development of underground nuclear waste repos itories the methodology is stochastic and incorporates geological information at scales ranging over several orders of magnitude the earthquake catalog from fennoscandia the tectonics and geomechanics of sweden and conservative numerical simulation of the impacts of earthquakes in sweden over the next 100 000 yr at the three study sites the slip predicted by the simulations is consistent with or conservative with respect to field data on distributed slip different measures of canister failure pre dicted at the three sites show how both differences in future seismicity and in the fracture geology impact canister failure the calculations also point out areas of uncertainty in the conceptual modeling of future seismicity and input data that may require additional work prior to using the method for licensing support we also describe applications of this methodology to other earthquake engineering problems

breaking new ground estimates of crack depth along the axial zone of the east pacific rise 9 12 54 n using simple fracture mechanics models the depths of fissures that were observed along the axial zone of the east pacific rise epr from 9 12 to 54 n with the deep towed argo z vehicle are estimated the widest cracks 4 8 m along this fast spreading portion of the epr are the deepest and are spatially correlated with the broadest youngest and most hydrothermally active portions of the ridge crest if the widest deepest cracks are not primarily eruptive they should be most abundant in areas of older lava flows having increased in width with time this is not observed some of the widest cracks are located where the 1991 eruption of the ridge crest occurred at 9 45 52 n and may be deep enough to reach the sheeted dikes of layer 2b these cracks may have tapped melt during the eruption and facilitated the flux of vapor rich hydrothermal fluids through overlying lava flows the narrowest presumably shallowest cracks correspond to the narrowest oldest and least hydrothermally active portions of the ridge crest we interpret the wide deep cracks as primarily eruptive in origin and suggest that they may be associated with the inflation of an axial magma chamber whereas the narrow shallow cracks are interpreted as primarily tectonic and are thought to be associated more with far field plate stresses

magmatism of the westernmost komandorsky segment of the aleutian island arc abstract four magmatic associations are distinguished in the westernmost komandorsky segment of the aleutian island arc basalt plagiorhyolite 45 28 ma trachybasalt teschenite 27 16 ma diorite granitoid 12 5 8 5 ma and dacite rhyolite 0 7 0 ma the latter association is represented by the lavas of a recently discovered active submarine volcano piip volcano about 40 km north of the komandorsky islands and 700 km west of the quaternary center on buldir is detailed petrogeochemical data including ree and 143 nd 144 nd and 87 sr 86 sr isotopic ratios show that the parental melts of the komandorsky is were of mantle origin and their sources remained isotopically homogeneous during the whole geological history of the aleutian arc 10be concentrations in lavas of the piip volcano 2 7 106 at g indicate subduction related incorporation of young pelagic sediments in their magmatic sources or contamination of ascending melts by non solidified sedimentary material directly beneath the volcano the komandorsky segment of the aleutian arc evolved from a depleted arc tholeiitic series through a markedly alkaline series to later calc alkaline andesites and dacites and finally to late stage calc alkaline dacites and rhyodacites erupted inboard of the older volcanic centers

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

petrogenesis of neoarchean metavolcanic rocks in changyukou northwestern hebei implications for the transition stage from a compressional to an extensional regime for the north china craton abstract the 2 5 ga metavolcanic rocks in changyukou northwestern hebei can be classified into three groups based on major and trace elements high mg basalts tholeiitic basalts and the calc alkaline series basaltic andesites andesites and dacites rhyolites both high mg basalts and tholeiitic basalts have negative anomalies of nb zr ti and heavy rare earth elements hree as well as enrichments of sr k pb ba and light rare earth elements lree and show typical subduction zone affinities the petrogenesis of high mg basalts can be ascribed to high degree partial melting of an enriched mantle source in the spinel stability field that was previously enriched in large ion lithophile elements lile and lree by slab derived hydrous fluids melts supercritical fluids as well as the subsequent magma mixing processes of different sources at different source depths with little or no influence of polybaric fractional crystallization the flat hree of tholeiitic basalts indicates they may also originate from the spinel stability field but from obviously shallower depths than the source of high mg basalts they may form at a later stage of the subduction process when rapid slab rollback leads to extension and seafloor spreading in the upper plate we obtain the compositions of the archean lower crust of the north china craton based on the archean wutai jining section by compiling the average tonalite trondhjemite granodiorite ttg components average mafic granulite components and average sedimentary rock components the modeling results show that the generation of high al basalts basaltic andesites and andesites can be attributed to assimilation by high mg basalts primary basalts of relatively high al 2 o 3 thickened lower crust and the subsequent crystallization of prevailing mafic mineral phases while al 2 o 3 rich plagioclase crystallization is suppressed under high pressure and nearly water saturated conditions dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites high al basalts and andesites mixing of magmas at various stages along the fractionation course of basaltic andesites high al basalts toward rhyolites promotes the trend of the calc alkaline series to reconcile the 2 55 to 2 5 ga ttgs derived from overthickened crust the 2 51 to 2 50 ga calc alkaline volcanic rocks derived from thickened crust tholeiitic basalts representing low pressure and an extensional tectonic setting 2493 ma leucosyenogranites derived from overthickened crust 2437 ma biotite monzogranites derived from slightly thinner crust than leucosyenogranites but still thickened as well as the clockwise hybrid itd and ibc p t paths of the hp granulites and widespread extension and rifting setting within the ncc from 2300 ma we propose a model of an evolving subduction process among them the composition of the 2 5 ga changyukou volcanic rocks and potassic granites as well as the clockwise hybrid itd and ibc p t paths of the hp granulites may reveal that the tectonic setting in northwest hebei was in a transition stage from a subduction related compressional regime to an extensional regime related to plate rollback

petrology and isotopic composition of quaternary basanites dredged from the bering sea continental margin near navarin basin quaternary basanites were recovered from shallow water depth from the continental margin of the bering sea 58 39 0 n 177 12 9 w near navarin basin the basanites are highly vesicular flow rock and hyaloclastites similar to other alkalic volcanic rocks erupted repeatedly during the late cenozoic on islands in the bering sea region and in mainland alaska k ar ages for the basanites indicate at least two episodes of volcanism at about 1 1 and 0 4 ma similar alkalic volcanism occurred sporadically at geographically widely separated centers in the bering sea region for at least the past 6 ma chemically these alkalic lavas are intraplate basalts similar to those erupted from oceanic islands and in some continental settings trace element data indicate these alkalic lavas have been generated by small but variable amounts of partial melting of a meta somatized lherzolite source the relatively primitive compositions mgo 9 presence of mantle derived xenoliths in some alkalic lavas and presence of f

andisol formation in a holocene beach ridge plain under the humid tropical climate of the atlantic coast of costa rica abstract soil formation has been studied in relation with time in a 5000 year old chronosequence on volcaniclastic beach ridges of the perhumid tropical atlantic coast of costa rica all soils are under tropical rainforest drainage conditions change by subsidence from excessively drained in the two youngest soils to imperfectly drained in the two oldest soils parent material is rather homogeneous andesitic sand with a volcanic glass component of less than 10 it has been found that under these conditions andisols form within 2000 years imperfect drainage caused mottling and accumulation of iron coatings as well as the formation of a thin o horizon in the oldest profiles sand content of the soils decreases regularly with soil age while the amount of fine material increases concurrently the increase in fine material and the accumulation of organic matter cause an increase of cec and andic properties and a decrease in bulk density and ph with soil age depth of biological influence increases with soil age but soil faunal activity is hampered in the oldest three profiles probably by imperfect drainage due to the extreme leaching conditions the sum of exchangeable cations is less than 2 cmol kg 1 in the b horizons of the older soils notwithstanding the presence of a considerable amount of weatherable primary minerals

tectonics of the thingvellir fissure swarm sw iceland abstract the thingvellir fissure swarm dissects 9000 year old pahoehoe lava and contains about 100 fractures of average orientation n29 3e the average length of fractures is 620 m the minimum being 57 m and the maximum 7 7 km the maximum width and throw on a single fracture are 68 m and 40 m respectively most fractures are vertical at the surface and must be the result of an absolute tensile stress the geometry and arrangement of the fractures indicate that they have grown by coalescence of initially offset small fractures it is concluded that most fractures attain depths of the order of several hundred meters or less but that the largest faults attain depths of many kilometers comparison with the vogar fissure swarm on the reykjanes peninsula suggests that the thingvellir swarm may have the greater rate of dilation the total maximum postglacial dilation of the vogar swarm is only 15 m whereas the corresponding figure for the thingvellir swarm is about 100 m

decompression and h2o exsolution driven crystallization and fractionation development of a new model for low pressure fractional crystallization in calc alkaline magmatic systems magma ascent decompression induced h2o exsolution and crystallization is now recognized as an important process in hydrous subduction zone magmas during the course of such a process calculations suggest that the ascent rate of a degassing and crystallizing mafic magma will be greater than crystal settling velocities thus any crystals formed as a consequence of volatile exsolution will remain suspended in the magma if the magma erupts before the percentage of suspended crystals reaches the critical crystallinity value for mafic magma 55 vol it will produce the commonly observed crystal rich island arc basalt lava if the magma reaches its critical crystallinity before it erupts then it will stall within the crust extension of compaction experiments on a 55 vol sand karo syrup suspension at different temperatures and liquid viscosities to the likely viscosities of interstitial andesitic to dacitic liquid within such a stalled magma suggest that small amounts up to 10 can be expelled on a time scale of 1 10 years the expelled liquid can create a new intermediate to silicic body of magma that is related to the original mafic magma via fractional crystallization the short time scale for liquid expulsion indicate that decompression induced h2o exsolution and crystallization can be an important mechanism for fractional crystallization based on this assumption a general model of decompression induced crystallization and fractionation is proposed that explains many of the compositional mineralogical and textural features of aleutian and other andesites

stress estimates from the length width ratios of fractures the hypothesis is advanced that provided young s modulus and poisson s ratio of the rock are known the length width ratios of tension fractures can be used to estimate the tensile stress assumed constant along the length of each fracture at the time of fracture formation the hypothesis is tested on a fissure swarm in a 10 000 year old basaltic lava in iceland the length width ratios of the fissures give the average tensile stress as of the order of a few mpa

widening of the krafla fissure swarm during the 1975 1981 volcano tectonic episode frequent distance measurements across the krafla fissure swarm north iceland recorded the extension accompanying the sequence of rifting events which started in december 1975 and lasted for 6 years an 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events maximum widening of about 8 m occurred 10 to 12 km north of leirhnjukur which is located above the center of the krafla magma reservoir from that location the amount of widening decreased north wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting off the north coast about 70 km north of leirhnjukur the amount of widening also decreased southwards and approached zero at 15 to 20 km south of leirhnjukur the ground deformation associated with these rifting events can be summarised as a narrow strip 1 to 2 km wide along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm this fractured strip has subsided 2 to 3 m relative to its flanks the flanks of the fractured zone have been uplifted relative to regions farther away the uplift is not well constrained but tilt observations at several locations indicate about 1 m uplift the flanks of the rift zone have contracted perpendicular to the fissure swarm the maximum contractional strain exceeds 300 mm per km the amount of areal expansion windening of the fissure swarm times the length of the fractured zone associated with these rifting events is estimated to be about 0 30 km2 for individual events the area of expansion has been roungly proportional to the volume of subsidence above the krafla magma reservoir if the width of a new dike is equal widening of surface fissures the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2 4 to 2 8 km this ratio is significantly less for events of large lava production but even during these events the majority of magma leaving the krafla reservoir was apparently emplaced in subsurface fissures

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

tholeiitic and calc alkaline magma series at adatara volcano northeast japan 2 mineralogy and phase relations abstract low alkali low k by gill 1978 or island arc by jakes and gill 1970 tholeiitic magma suites are closely associated with medium k by gill 1978 calc alkaline suites in the outer volcanic zone called nasu volcanic zone nvz of northeast japan chemistry and phase relations of phenocrysts are studied for representative 5 low alkali tholeiitic and 8 calc alkaline rock samples of adatara volcano in nvz in order to examine the mineralogical evolutionary mechanisms of the two series of magmas also clarified are differences in the magmatic temperature and oxygen fugacity between the two magma series the correlations between whole rock chemical compositions and mineralogy in the tholeiitic suite at this volcano are well documented supporting the argument that the members of this suite are related through fractional crystallization of observed phenycrystic phases part 1 fujinawa 1988 mineralogical observations in the calc alkaline suite are compatible with the views based on their whole rock chemical compositions although fractional crystallization process is dominant in the evolution of the calc alkaline magma intermittent mixing of small amounts of magnesian high temperature magmas operates concurrently temperature estimates with the two pyroxenes geothermometer range from 1080 c to 970 c in the tholeiitic suite whereas from 990 to 850 c in the calc alkaline suite temperature oxygen fugacity trends indicate that the low alkali tholeiitic magma evolves under conditions of higher temperature and lower oxygen fugacity than the calc alkaline magma in view of the fact that the hyperstene and augite phenocryst cores in the calc alkaline samples are compositionally distinguishable from the relevant phases in the tholeiitic samples it is unlikely that magma mixing played an essential role in the generation of the calc alkaline suite at adatara volcano

the influence of amphibole fractionation on the evolution of calc alkaline andesite and dacite tephra from the central aleutians alaska petrologic studies of tephra from kanaga adak and great sitkin islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc alkaline andesite and dacite magmas in the central aleutians amphibole is ubiquitous in tephra from kanaga and adak islands whereas it is present only in a basaltic andesite pumice from great sitkin dacitic tephra from great sitkin do not contain amphibole hornblende dacite tephra contain hb plag ox opx cpx phenocrysts with simple zoning patterns suggesting that the dacites evolved in isolated magma chambers andesitic tephra from adak contain two pyroxene and hornbelende populations and reversely zoned plagioclase indicating a more complex history involving mixing and fractional crystallization mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole bearing cumulate xenoliths both formed during the evolution of high al basalts the presence of amphibole in andesitic and dacitic tephra implies that aleutian cale alkaline magmas evolve in the mid to lower crust under hydrous 4 wt h2o and oxidizing ni nio conditions amphibole bearing andesites and pyroxene bearing dacites from great sitkin indicates fractionation at several levels within the arc crust despite its absence in many calc alkaline andesite and dacite lavas open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on adak island neither open nor closed system fractionation involving a pyroxene bearing assemblage is capable of explaining the trace element concentrations or ratios found in the adak suite we envision a scenario where amphibole was initially a liquidus phase in many calc alkaline magmas but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust the mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma whereas the trace element chemistry provides evidence for a earlier amphibole bearing mineral assemblage

an experimental investigation of volatile exsolution in evolving magma chambers abstract previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas turbulent transfer of heat between a layer of dense hot and volatile rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing followed by further exsolution of gas and explosive eruption these processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of co 2 we have investigated both the initial overturning produced by gas release in the lower layer and the subsequent evolution of gas due to intimate mixing of the two layers the latter experiments in which the reactants remained isolated in the two layers until overturning occurred demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat this shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other the magmas can coexist in the same stratified chamber until their bulk densities become equal and they mix together the processes illustrated in these experiments could occur in h 2 o bearing magmas such as in the calcalkaline association and in co 2 bearing mafic magmas such as in silica undersaturated suites

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

large glass inclusions in plagioclase phenocrysts and their bearing on the origin of mixed andesitic lavas at toliman volcano guatemala irregularly shaped large and clear lac glass inclusions are present in plagioclase phenocrysts in several andesitic lavas erupted from toliman volcano guatemala their morphology is different from densely spaced fine grained glass inclusions that form concentric zones in dusty or cellular textured plagioclase phenocrysts the large size of lac inclusions make them suitable for microprobe analysis and average bulk compositions are presented for glasses in 30 phenocrysts from eight lava samples their compositions are rhyolitic and in disequilibrium or out range anderson 1976 with respect to whole rock and groundmass glass compositions lac inclusions typically occur in large tabular plagioclase phenocrysts with relatively uniform sodic compositions an 40 54 compositions of feldspar phenocrysts not containing lac inclusions range from an 41 to an 81 petrographic and chemical data support a primary origin for lac glasses suggest mixing of mafic and silicic magmas and also constrain a mechanism for magma mixing rapid growth of plagioclase and entrapment of lac glass occurs during mixing in a vapor rich silicic liquid under low degrees of undercooling these conditions are possibly produced in a high level magma body such as that envisioned by huppert et al 1982 where replenishment and subsequent crystallization of a hydrous magma induces density instability and mixing with the resident magma

the origin of tensile fracture lineaments abstract fracture controlled lineaments commonly seen where brittle basement is exposed at the earth s surface are generally restricted to a small number of sets with angles of 45 90 between sets the length frequency distribution of lineaments in each set follows a truncated poisson function such lineaments usually show almost no shearing offset suggesting a tensile origin a simple mechanical model of tensile fracturing is used to explain the spacing directions and length of lineaments as well as their depth frequency distribution results suggest that the penetration depth of tensile fractures which produce lineaments at the earth s surface is directly related to their length and that the fracture density is inversely proportional to fracture depth finally the angles between lineament sets may be controlled by the ratio of strength of unfractured rock to that of pre existing fractures which might heal with time the most likely source of tension is tectonic uplift fractures due to typical uplifts of 0 5 1 km over distances of 10 100 km may penetrate as brittle fractures to several kilometres into the crust perhaps to the depth at which seismic activity ceases

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

evolution of migrating transform faults in anisotropic oceanic crust examples from iceland major transform fault zones link extensional segments of the north american eurasian plate boundary as it transects the iceland hotspot changes in plate boundary geometry involving ridge jumps

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

quantifying arc migration and the role of forearc subduction erosion in the central aleutians abstract the active aleutian arc is currently classified as an accreting margin as most of the arc has a well developed frontal accretionary prism that formed in the plio pleistocene we focus on the central aleutian island arc where the arc massif was built to near its current size by the late eocene and has since seen limited growth and endured episodic erosion prior to the development of the accretionary prism as a result the volcanic front has migrated northwards tens of kilometers since inception along with previous mapping dating and geochemical data sixteen new 40ar 39ar dates and new geochemical data for volcanic rocks on several central aleutian islands supplement recent geochronologic data from six other islands in this region and facilitate an attempt to understand how and where aleutian arc volcanism has migrated with time long term average arc migration rates range from 0 6 to 5 km ma in three distinct locations the arc migration rate increases to the west and is greatest over the last five million years as the arc migrated from a 7 5 ma volcanic front to its current location arc migration and erosion during this most recent period may have been expedited by either the subduction of the kula ridge which ceased spreading in the early eocene or clockwise rotation of the pacific plate in the late miocene some central aleutian lavas and plutonic rocks have elevated la yb sm yb and sr yb most of which were emplaced during times of arc migration or in the waning stages of a peak in magmatic activity the strongly fractionated trace element patterns are similar to the basaltic andesites to dacites from the western aleutian seafloor which are interpreted to be the result of residual garnet during partial melting of eclogite in the subducting oceanic crust however the existing subduction parameters coupled with the timing of the central aleutian adakite like lavas favors a model in which significant subduction erosion of the mafic forearc including erosion and subsequent melting of the trapped kula plate i e morb like oceanic crust as well as early aleutian arc basalts in the mid crust may be partly responsible for the eclogitic component observed in central aleutian magmas

the evolution of high alumina basalts of the klyuchevskoy volcano kamchatka russia based on microprobe analyses of mineral inclusions abstract the origin of calc alkaline high alumina basalts hab of the klyuchevskoy volcano kamchatka was examined using electron microprobe analyses of phenocrysts and mineral phases included in the phenocrysts continuous trends on major element variation diagrams suggest the hab were derived from high magnesia basalt hmb by fractional crystallization phenocrysts in the hab are strongly zoned olivine mg 91 64 clinopyroxene wo45 38en40 51fs5 20 and chrome spinel magnetite inclusions in them cr2o3 45 0 wt tio2 0 5 11 microprobe analyses of minerals included in the phenocrysts provide additional constraints on the mineral crystallization trends in the hab fe mg partitioning data when applied to the phenocrysts cores show they crystallized from a hmb the similarity of phenocryst core compositions in hab with those in hmb strongly suggests a genetic relationship between the two magma types

diffuse faulting in the machu picchu granitoid pluton eastern cordillera peru abstract a series of batholiths forming part of the roots of a permo liassic rift system are exposed in the high eastern cordillera of central peru as a result of tectonic inversion shortening of the machu picchu granitoid pluton was accommodated by widespread shear reactivation of primary joints by a process termed here diffuse faulting fault like reactivation of precursor joint surfaces marked by chlorite epidote and quartz shear fibres is locally evidenced by few centimetres offsets within apparently undeformed granite analysis of fault slip data indicates that shear reactivation of different joint sets was kinematically consistent with ene oriented shortening less frequent mylonitic shear zones appear to have evolved from the common brittle precursors apart from rare phyllonitic shear zones fluid rock interaction along the brittle precursors was generally limited and pluton deformation appears to be mainly controlled by the geometry and distribution of primary joints three main sets of reactivated joints can be recognized characterized by oblique slip kinematics with variable reverse and strike slip components of motion theoretical modelling based on quantitative fracture analysis scan line data and different displacement length relationships applied to the main reactivated joint sets yield first order estimates of pluton finite strain the results suggest that bulk finite strain is oblate and essentially coaxial and is characterized by horizontal shortening not exceeding 10 relatively small finite strains integrated over the size of the pluton still result in a few kilometres of crustal shortening

clay minerals of four soils formed in eolian and tephra materials in iceland abstract clay minerals in a vitricryand and three haplocryands derived from eolian and tephra materials in different parts of iceland and in glacial till samples representative of underlying strata were studied by a combination of methods the soils are primarily used as rangelands and are highly susceptible to wind erosion allophanes imogolite and poorly crystalline ferrihydrite are abundant 700 g kg of the clay throughout the soils including a horizons this clay mineralogy would give rise to stable low density aggregates that influence the susceptibility of the soils to wind erosion it also affects chemical properties such as high phosphate retention of the soils even of the cryands with relatively low clay contents 160 275 g kg imogolite was found in some but not all horizons of all soils no particular relationship of imogolite to measured si al molar ratios of the oxalate oxalic acid soluble fractions of the clays 0 51 to 1 13 was evident layer silicate minerals and laminar opaline silicas are either not present or occur in small quantities plagioclase augite and or their weathering residues were found in the clay and silt separates dark colored glass is predominant in the fine sand throughout the soils and the tills one till sample had a clay mineral composition similar to the eolian andic soils while the other contained smectite and possibly a spherical halloysite smectite intermediate but not allophane and imogolite as major weathering products the clay mineral composition of these soils and tills was compared with those derived from basic tephras in other regions

eruptive history and petrology of mount drum volcano wrangell mountains alaska mount drum is one of the youngest volcanoes in the subduction related wrangell volcanic field 80x200 km of southcentral alaska it lies at the northwest end of a series of large andesite dominated shield volcanoes that show a northwesterly progression of age from 26 ma near the alaska yukon border to about 0 2 ma at mount drum the volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka cone lavas range from basaltic andesite to dacite in composition ring domes are dacite to rhyolite the last constructional activity occured in the vicinity of snider peak on the south flank of the volcano where extensive dacite flows and a dacite dome erupted at about 250 ka the climactic explosive eruption that destroyed the top and a part of the south flank of the volcano produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows the mount drum rocks have medium k calc alkaline affinities and are generally plagioclase phyric silica contents range from 55 8 to 74 0 wt with a compositional gap between 66 8 and 72 8 wt all the rocks are enriched in alkali elements and depleted in ta relative to the lree typical of volcanic arc rocks but have higher mgo contents at a given sio2 than typical orogenic medium k andesites strontium isotope ratios vary from 0 70292 to 0 70353 the compositional range of mount drum lavas is best explained by a combination of diverse parental magmas magma mixing and fractionation the small but significant range in 87sr 86sr ratios in the basaltic andesites and the wide range of incompatible element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas the lavas show abundant petrographic evidence of magma mixing such as bimodal phenocryst size resorbed phenocrysts reaction rims and disequilibrium mineral assemblages in addition some dacites and andesites contain mg and ni rich olivines and or have high mgo cr ni co and sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite but the rhyolites have k ba th and rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks limited sr isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks

experimental constraints on the effects of pressure and h 2 o on the fractional crystallization of high mg island arc basalt h2o undersaturated melting experiments of synthesized basalt sio2 50 7 wt mgo 8 3 wt mg 60 were conducted at fo2 corresponding to nno 1 and nno 1 to clarify the effects of pressure 2 7 kbar and h2o on fractional crystallization in island arcs h2o content was ranged from nominally anhydrous to 4 4 wt differentiation trends namely the liquid lines of descent change sensitively according to pressure h2o relations tholeiitic differentiation trends are reproduced with h2o 2 wt in primary magma with such quantities of h2o fractional crystallization is controlled by olivine plagioclase at 2 kbar increasing the pressure from 2 to 4 kbar induces early crystallization of orthopyroxene instead of olivine and therefore sio2 enrichment in the residual melts is suppressed increasing h2o 2 wt in primary magma stabilizes clinopyroxene relative to orthopyroxene and or magnetite although the phase relations and proportions strongly depend on fo2 and h2o content differentiation trends are always calc alkaline

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

brittle deformation and fracture patterns on oceanic rift shoulders the esja peninsula sw iceland abstract this paper provides detailed structural data on the crustal deformation of the esja peninsula at the southwestern margin of the rift zone in sw iceland forty percent of the faults are strike slip with steep dips 35 are normal dip slip with dips ranging from 50 to 90 for normal faults e w ne sw and nne ssw are the three major trends the paleostress tensors determined using the fault data indicate three main states of stress two extensional regimes 3 n180 e 10 and n110 e 10 are related to normal slip faults the third is a compressive stress regime with 1 trending n10 e n30 e associations between strike slip and dip slip faults are often observed resulting from both the geometrical requirements of interactions between faulted blocks and from local regional permutations between 1 and 2 we note a constant obliquity of 10 20 between the trend perpendicular to the regional structures and the computed direction of extension this conclusion is also supported by the major trends of dykes and tensional fractures there are two mechanisms of volcanic injection local sheets and regional dykes resulting in power law cumulative thickness distributions with exponents of 1 5 and 3 0 respectively

the evolution of crustal deformation in an oceanic extensional environment abstract fault statistics collected along the slow spreading reykjanes ridge are used to investigate its complex tectonic evolution the reykjanes ridge is oriented oblique to the spreading direction and is located close to the iceland hotspot a wide distribution in fault strike and fault throw occurs across the axis of the reykjanes ridge with a sharp change in mean strike and mean throw occurring at the edge of the axial zone within the axial zone there is no significant change in the distribution of faults with increasing distance from iceland proximity to the hotspot does not influence the detailed tectonics near the axis we infer that the fault development within the axial zone is controlled by the plate tectonic generated regional stress field outside the axial zone the faults strike oblique to the spreading normal direction this change in fault strike coupled with the lack of any evidence for strike slip movement suggests that these faults develop as a passive response to the thermal evolution of the lithosphere our results indicate that at a slow spreading ridge the crust is deformed by two stress fields one responsible for the fault formation within the axial zone the other for the faults which bound the axial zone

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

basalt convection and plagioclase retention a model for the generation of high alumina arc basalt recent investigations of crystal settling in convecting magma have suggested that crystals may be spatially sorted according to both size and mineralogy for a basalt magma simultaneously crystallizing plagioclase clinopyroxene and olivine a possible result is the selective removal of mafic minerals and the preferential retention of plagioclase crystals along the intrusion margin this represents a possible mechanism for generating plagioclase phyric magmas of high alumina basalt composition in this study a combined basalt crystallization and convection model has been developed to assess the extent to which crystallization and convection leads to mineralogic sorting and preferential plagioclase retention in a primitive mantle derived calc alkaline basalt intrusion the model assumes a simple rectangular chamber geometry steady state non turbulent single cell convection and spatially uniform crystallization the results indicate that over a narrow range of convective velocities 1 crystal sorting

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico i disequilibrium phenocryst assemblages as indicators of magma chamber processes

pre eruptive magma mixing and crystal transfer revealed by phenocryst and microlite compositions in basaltic andesite from the 2008 eruption of kasatochi island volcano the august 7 8 2008 eruption of kasatochi island volcano located in the central aleutians islands alaska produced abundant compositionally heterogeneous basaltic andesite 52 55 wt sio2 that has been interpreted to result from pre eruptive magma mixing the basaltic andesite contains two populations of plagioclase phenocrysts the first volumetrically dominant population consists of oscillatory zoned phenocrysts with an overall normal zonation trend toward comparatively sodic rims an55 65 interrupted by dissolution features and spikes in calcium content up to an85 the second population consists of phenocrysts with highly calcic compositions an90 these phenocrysts contain sharp decreases in calcium content close to their rims reaching as low as an60 but are otherwise texturally and compositionally homogeneous groundmass plagioclase microlites are generally much more calcic than rims of the first phenocryst population with more than 50 of measured microlites containing an80 major minor and trace element concentrations of plagioclase microlites and phenocrysts indicate that oscillatory zoned phenocrysts derived from cooler 800 950 c more silicic mixing magma while unzoned calcic phenocrysts were associated with hotter 900 1050 c mafic magma the mixing of these magmas just prior to eruption followed by decompression during the eruption itself created high effective undercoolings in the mafic end member and lead to the nucleation of high an microlites mgo and feo concentrations of plagioclase microlites and high an phenocryst rims up to 0 4 and 1 3 wt respectively provide further evidence for high mixing and eruption induced effective undercoolings

intracrustal controls on the coexistence of tholeiitic and calc alkaline magma series at aso volcano sw japan aso caldera sw japan was formed incrementally over a period of 300 kyr by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits aso 1 4 these units are collectively referred to as the aso pyroclastic flow deposit apfd in which tholeiitic and calc alkaline magmas have been erupted in close temporal and spatial proximity detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between aso 1 and 4 as the caldera forming stage evolved changing from a predominantly tholeiitic to a calc alkaline signature the mineralogical and geochemical characteristics of aso 1 4 suggest that magmatic evolution within each unit is controlled by fractional crystallization associated with magma mixing crustal assimilation by altering the fractional crystallization mineral assemblage as well as the relative amounts of fractional crystallization magma mixing and crustal assimilation the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced this suggests that the coexistence of tholeiitic and calc alkaline magma series can be attributed to shallow level intracrustal processes

weathering and soil forming processes under semi arid conditions in two mexican volcanic ash soils abstract weathering and neoformation of allophane imogolite and halloysite in volcanic ash soils have been studied extensively in humid climates but little data are available on these soils in arid and semi arid conditions weathering and soil formation under semi arid conditions on acid pyroclastics have been investigated in two profiles both at the micro site scale and on bulk samples xr microdiffractions and sem edx microanalysis were performed on soil thin sections chemical particle size xrd and mineral identification analysis were performed on conventional soil samples results show the interplay of various sub processes a intense division of the coarse minerals into small fragments 50 200 m in size b diagenesis of noncrystalline products in the zone of contact with the parent minerals c transformation of noncrystalline minerals into halloysite in the compact soil microstructures and preservation of the noncrystalline minerals in the topsoil d desiccation and condensation of the crystalline and noncrystalline minerals into microaggregates in the topsoil the results of these interacting weathering processes are silty loam soils with no cohesion and high susceptibility to wind erosion differences appearing between different analytical methods at different sampling scales need special precautions in explaining the results

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

on the na2o content of convergent zone high alumina basalts abstract high alumina basalts in the convergent zone are classified into high medium and low na type the high na high alumina basalts occur in arcs without low alkali tholeiites where younger and warmer oceanic lithospheres are subducting the low na high alumina basalts occur in arcs where on the contrary older and cooler plates are descending and occur also in arcs with extensional tectonics even though younger lithospheres are underthrusting a good correlation is observed between the na2o content of high alumina basalts and the age of the subducting lithosphere this suggests that generation of high alumina basalt magma is controlled by the nature of the descending plate

probing the evolving andean lithosphere mid late tertiary magmatism in chile 29 30 30 s over the modern zone of subhorizontal subduction in the mid late tertiary 27 6 m y ago volcanism diminished then ceased in the main andean cordillera in chile between 29 and 30 30 s over the same time span the subduction angle shallowed under this region which today lies in the modern shallowly dipping nonvolcanic segment of the south central andes 28 33 s the trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition volcanic changes include in addition to a decreased rate of volcanism a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks and an increase in crustal assimilation these volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space time pattern of deformation and volcanism in the foreland region although the model stands alone support for the tectonic setting and magmatic source region of these tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern andean segments the older dona ana formation 27 18 m y is similar to volcanic rocks in the southern volcanic zone while the younger cerro de las tortolas 14 10 m y and vallecito 6 m y formations are similar to volcanic rocks over regions of thicker crust in the central volcanic zone and the northern part of the southern volcanic zone

pedogenic development of volcanic ash soils along a climosequence in northern taiwan abstract andisols have been shown to evolve over time to other more weathered soil types however more information is needed on the time scales over which these changes occur under different climatic conditions in this study we analyze mineral weathering and soil formation along a soil climosequence formed on volcanic deposits of 400 ka of age in the subtropical climate of northern taiwan we sampled 6 pedons at elevations between 140 and 1090 m above sea level asl representing a climatic gradient from about 22 c mean annual temperature mat and 2000 mm mean annual precipitation map to about 16 5 c mat and 5000 mm map our results show that phosphate retention al o 1 2fe o si o and the fe o fe d ratio significantly increased with elevation the high elevation soils 900 m asl contained metastable poorly crystalline materials e g allophane ferrihydrite and classified as andisols whereas the low elevation soils

evolution of mantle derived augite hypersthene granodiorites by crystal liquid fractionation barrington tops batholith eastern australia the barrington tops batholith btb is a shallow level intrusive complex of permian age 262 ma in zone a of the new england fold belt northern new south wales it comprises a comagmatic suite of early pyroxene plagioclase phyric quartz diorite stocks and dykes 10 by area augite hypersthene granodiorite 75 and aplite less voluminous hornblende biotite granodiorite 15 appears to be related to the augite hypersthene granodiorite by the addition of minor amounts of an incompatible element rich component the augite hypersthene granodiorite plutons formed by fractional crystallisation of a dry 2 0 high temperature 1000 c liquid major element modelling using analysed phenocryst compositions closely reproduces the geochemical differentiation trends observed combined petrographic and mineral chemistry evidence indicates that parental liquids were of quartz dioritic and more mafic compositions probably mantle derived low initial 87 sr 86 sr 0 7036 0 7039 and high nd values 5 6 to 7 8 suggest these were produced from a geochemically depleted mantle source the btb may be regarded as an end member of the spectrum of calc alkaline granitic rocks that are formed by the mixing of variably fractionated melts of upper mantle origin with lower continental crust material it is suggested that lower crustal mafic and ultramafic xenoliths found m mesozoic to cenozoic basalts represent cumulates complementary to the more evolved mantle derived component of calc alkaline magmatism crystal liquid fractionation aided by mixing of evolved mantle derived magma with crustal material appears to be the most important process giving rise to the diverse geochemical and isotopic continuum displayed by calc alkaline magmatism m orogemc terrains

dyke emplacement in a layered and faulted rift zone abstract in rift zone eruptions magma is normally transported to the surface through dykes most injected dykes however do not make it to the surface but rather become arrested at certain depths in the rift zone surface deformation studies are routinely used to infer the depths to the tips as well as the geometries of arrested dykes most such studies model the rift zone as a homogeneous isotropic elastic half space all rift zones however contain normal faults and are composed of rock layers with different mechanical properties the numerical models presented here indicate that when the upper tip of a vertically propagating dyke reaches the same crustal level as the bottom tips of the boundary faults of an active graben the faults close as the tip continues its propagation up into the graben the overpressured dyke encourages reverse slip on the boundary faults the present numerical models also indicate that mechanical layering has strong effects on dyke emplacement and the surface deformation associated with arrested dykes in rift zones in particular soft low young s modulus layers and weak contacts at shallow depths contribute to offset arrest and aperture thickness variations of the dykes and suppress the dyke induced surface stresses and associated brittle deformation when the magmatic overpressure of a dyke is the only loading the dyke tends to be thicker inside soft layers than inside stiff high young s modulus layers soft layers and weak contacts discontinuities common at shallow depths in rift zones suppress dyke tip tensile stresses thereby encouraging dyke arrest and may result in little if any surface stresses and brittle deformation being induced by an arrested dyke weak contacts also encourage transfer of the dyke induced surface tensile stresses to the regions above the lateral ends of the contacts then the tensile stresses at the rift zone surface in a large area above the dyke itself become small or negligible all the tensile stress occur above the lateral ends of the contact which may be many kilometres from the dyke tip for a dyke arrested in a layered rift zone straightforward inversion of surface deformation data may therefore yield geometric results that have little relation to the actual geometry of the arrested dyke the results of the numerical models presented here are generally supported by field observations of dykes in rift zones

clinopyroxene phenocrysts with green salite cores in trachybasalts implications for two magma chambers under the kokchetav uhp massif north kazakhstan abstract clinopyroxene phenocrysts in the kokchetav trachybasalts are variable in composition and textures two distinctive cores are recognized diopside cores and green salite cores the diopside cores with mg of 80 90 are mantled by colorless salite rims with mg of 70 80 the green salite cores have especially low mg

the 7 8 august 2008 eruption of kasatochi volcano central aleutian islands alaska 1 kasatochi volcano in the central aleutian islands erupted unexpectedly on 7 8 august 2008 kasatochi has received little study by volcanologists and has had no confirmed historical eruptions the island is an important nesting area for seabirds and a long term biological study site of the u s fish and wildlife service after a notably energetic preeruptive earthquake swarm the volcano erupted violently in a series of explosive events beginning in the early afternoon of 7 august each event produced ash gas plumes that reached 14 18 km above sea level the volcanic plume contained large amounts of so2 and was tracked around the globe by satellite observations the cumulative volcanic cloud interfered with air travel across the north pacific causing many flight cancelations that affected thousands of travelers visits to the volcano in 2008 2009 indicated that the eruption generated pyroclastic flows and surges that swept all flanks of the island accumulated several tens of meters of pyroclastic debris and increased the diameter of the island by about 800 m pyroclastic flow deposits contain abundant accidental lithic debris derived from the inner walls of the kasatochi crater juvenile material is crystal rich silicic andesite that ranges from slightly pumiceous to frothy pumice fine grained pyroclastic surge and fall deposits with accretionary lapilli cover the lithic rich pyroclastic flow deposits and mark a change in eruptive style from episodic explosive activity to more continuous ash emission with smaller intermittent explosions pyroclastic deposits completely cover the island but wave erosion and gully development on the flanks have begun to modify the surface mantle of volcanic deposits

cerro quemado guatemala the volcanic history and hazards of an exogenous volcanic dome complex abstract cerro quemado is a holocene andesitic to dacitic exogenous dome complex located in a back arc volcanic field 5 km west of quezaltenango guatemala activity in the back arc almolonga volcanic field began prior to 84 000 yr b p with growth and collapse of an andesitic stratovolcano and has continued to the present with sporadic eruption of silicic domes and flows the cerro quemado dome complex covers about 12 km 2 has a volume of nearly 2 km 3 and comprises at least 8 vents historic activity is limited to extrusion of a blocky lava flow during a six month period beginning in january 1818 cerro quemado lavas are biotite bearing hornblende andesite and dacite the calc alkaline lavas have a disequilibrium mineral assemblage that includes three populations of zoned plagioclase phenocrysts and microphenocrysts hornblende biotite magnetite two pyroxenes olivine and quartz in addition spherical inclusions 2 mm to 30 cm of basaltic andesite sio 2 56 wt are ubiquitous in cerro quemado lavas this petrographic evidence points to an origin by mixing of felsic and mafic magma for cerro quemado in the past cerro quemado typically erupted viscous blocky lava flows up to 120 m thick and 2 5 km long and plug domes the presence of tephra and pyroclastic flow deposits however indicates that explosive volcanism has also occurred at the dome about 1150 yr b p the west flank of the dome gravitationally collapsed producing a debris avalanche that underlies 13 km 2 of the llano del pinal valley and avalanche caldera 1 km 1 5 km 90 m deep the caldera and avalanche deposit have identical volumes of approximately 0 1 km 3 the collapse apparently precipitated a hot laterally directed blast that swept about 40 km 2 of rugged terrain that includes llano del pinal valley and the east slopes of nearby siete orejas to an elevation of 3100 m a pyroclastic flow overlying the fines poor lateral blast deposit apparently resulted from remobilization and flow age of hot blast deposits off the steep slopes of siete orejas subsequently a small peleean dome was emplaced in the upper part of the avalanche caldera following the catastrophic events of 1150 yr b p cerro quemado was dormant until 1818 a d geologic mapping at cerro quemado volcano indicates that the principal volcanic hazards includes lava flow and exogenous dome growth pyroclastic flow debris avalanche lateral blast tephra fall and mudflow if cerro quemado erupts in the future quezaltenango and environs population more then 125 000 will be threatened

effects of volatiles on mixing in calc alkaline magma systems the intimate mixing between different magmas of disparate densities characteristic of some calc alkaline magma systems is explained by a new mechanism involving the emplacement of a layer of wet undersaturated mafic magma at the base of a magma chamber containing more differentiated magma turbulent transfer of heat between the mafic magma and the overlying magma leads to crystallization and exsolution of volatiles in the lower layer for an initial water content of a few per cent the bulk density of the mafic magma can become equal to that of the overlying magma causing overturning and intimate mixing of the magmas the mechanism is most probable in the low pressure environment of a high level magma chamber

soils developed on volcanic materials in the island of thera greece samples were taken of soils formed on volcanic material primarily composed of rhyodacitic pumice on the island of thera greece south aegean sea southeastern mediterranean and analyses for physical and chemical properties carried out these soils are characterised as having a coarse texture low organic matter content low cation exchange capacity cec and soil moisture retention but a high ph despite the acidic nature of the parent material extraction with sodium dithionite citrate acid ammonium oxalate sodium pyrophosphate and hot 0 5 m naoh showed that in contrast to soils usually forming on volcanic material worldwide soils lack noncrystalline minerals like allophane imogolite ferrihydrite and iron and aluminium humus complexes the only noncrystalline mineral present and in great quantities is amorphous silica winds prevailing practically all year round transfer droplets of seawater to these soils which in combination with the great porosity relatively high sodium content of the parent material and low rainfall are the primary factors in the poor development of these soils and high ph no andic properties are apparent and hence these soils cannot be classified as andisols the major factors determining soil genesis on the island of thera would appear to be the nature of the parent material and the climate d 2004 elsevier b v all rights reserved

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

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

petrology and age of volcanic arc rocks from the continental margin of the bering sea implications for early eocene relocation of plate boundaries eocene volcanic flow and dike rocks from the beringian margin have arc characteristics implying a convergent history for this region during the early tertiary the extrusive rocks are basalt basaltic andesite andesite and minor dacite and rhyolite the intrusive sample is from a quartz diorite dike intruding serpentinized peridotite major element oxide contents particularly feo mgo versus sio2 identify both tholeiitic and calc alkalic basalt more silicic lavas have calc alkalic affinities consistent with volcanic arc compositions spidergrams show pronounced nb ta depletion and alkali enrichment relative to light rare earth element lree abundance chondrite normalized ree plots show relatively flat patterns with only slight lree enrichment for tholeiitic compositions and greater lree enrichment and lower heavy rare earth element hree abundance for calc alkalic compositions the samples particularly those with calc alkalic compositions are rich in plagioclase that is strongly zoned the mo

surface deformation in volcanic rift zones abstract the principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes some of which feed fissure eruptions elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift if eruption occurs the characteristic ridge trough ridge displacement profile changes to a single ridge centered at the fissure and the erupted lava alters the local topography a well developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events to investigate this process we compute the elastic displacements and stresses in a homogeneous two dimensional half space driven by a pressurized crack that may breach the surface a derivative graphical method permits one to estimate the three geometric parameters of the dike height inclination and depth to center and the mechanical parameter driving pressure rock stiffness from a smoothly varying displacement profile direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation notably normal faulting creates discontinuities in the profile geological structures open cracks normal faults buckles and thrust faults form because of stresses induced by dike emplacement and fissure eruption theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation

the cold bay volcanic center aleutian volcanic arc the cold bay volcanic center has experienced two major stages of eruptive activity early m series acitivity produced bimodal hi alumina basalt and calc alkaline andesite lavas while later fpk series activity produced only calc alkaline andesite the spectrum of basalt compositions is believed to be due to high pressure 8 kb fractionation at or near the base of the crust abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites inspection of the mineralogical data has shown that the earliest m series andesites were produced by mixing of basalt 53 wt sio2 and silicic andesite 60 5 to 62 5 wt while later fpk series andesites resulted from the mixing of basaltic andesite 53 to 56 wt and less silicic andesite 58 5 to 60 0 wt the major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions the complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and or assimilation role in the origin of the silicic endmembers many of the features of all andesites including the important long term convergence of endmember magma compositions are consistent with the process of liquid fractionation accompanied by large scale magma mixing a deduced upper limit of 62 5 wt sio2 for the silicic endmember magmas suggests that liquid fractionation in the absence of major crustal melting cannot produce more silicic magmas a possible explanation is the presence of a rheological barrier based on the concept of critical crystallinity marsh 1981 which prohibits more silicic liquids from being extracted from a crystal liquid suspension

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

geochemistry and petrogenesis of early proterozoic amphibolites west central colorado u s a abstract early proterozoic supracrustal rocks exposed in west central colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments three successions are recognized the dubois succession 1770 1760 ma is comprised chiefly of amphibolites and felsic volcanics the amphibolites exhibit low concentrations of light ree and hfse and are broadly similar in composition to tholeiites from immature island arcs and in some respects to nmorb the cochetopa succession 1745 1730 ma includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible element enrichment similar to basalts from evolved island arc systems and associated back arc basins the black canyon succession is coeval with the cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite these amphibolites exhibit significant incompatible element enrichment similar to basalts from continental margin arc systems geochemical modeling indicates that the amphibolites evolved by closed system fractional crystallization of depleted for the dubois succession to variably enriched parental basalts for the cochetopa and black canyon successions chiefly at shallow depths incompatible element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted 1770 1760 ma to variable but slightly enriched 1745 1730 ma a provisional model calls upon the dubois succession forming in an island arc system followed by collision with the archean wyoming province at 1750 ma the cochetopa succession was deposited in an intra arc basin developed within a new continental margin arc and the black canyon succession formed in a back arc basin associated with this arc

volcanism and hydrothermalism on a hotspot influenced ridge comparing reykjanes peninsula and reykjanes ridge iceland abstract current estimates indicate that the number of high temperature vents one of the primary pathways for the heat extraction from the earth s mantle at least 1 per 100 km of axial length scales with spreading rate and should scale with crustal thickness but up to present shallow ridge axes underlain by thick crust show anomalously low incidences of high temperature activity here we compare the reykjanes ridge an abnormally shallow ridge with thick crust and only one high temperature vent known over 900 km axial length to the adjacent subaerial reykjanes peninsula rp which is characterized by high temperature geothermal sites confined to four volcanic systems transected by fissure swarms with young holocene volcanic activity multiple faults cracks and fissures and continuous seismic activity new high resolution bathymetry gridded at 60 m of the reykjanes ridge between 62 30 n and 63 30 n shows seven axial volcanic ridges avr that based on their morphology geometry and tectonic regime are analogues for the volcanic systems and fissure swarms on land we investigate in detail the volcano tectonic features of all mapped avrs and show that they do not fit with the previously suggested 4 stage evolution model for avr construction instead we suggest that avr morphology reflects the robust or weak melt supply to the system and two or more eruption mechanisms may co exist on one avr in contrast to 4 stage evolution model our interpretations indicate that unlike on the reykjanes peninsula faults on and around avrs do not cluster in orientation domains but all are subparallel to the overall strike of avrs orthogonal to spreading direction high abundance of seamounts shows that the region centered at 62 47 n and 25 04 w between avr 5 and 6 is volcanically robust while the highest fault density implies that avr 1 and southern part of avr 6 rather undergo period of melt starvation based on our observations and interpretations we expect all of the avrs on reykjanes ridge to be hydrothermally active but morphological and hydrographic settings of this ridge may cause hydrothermal plumes to be quickly dispersed and diluted due to exposure to strong bottom currents therefore combined ctd and autonomous vehicles surveys are probably the most efficient methods for hydrothermal exploration along the reykjanes ridge