crystallisation and zircon saturation of calc alkaline tonalite from the adamello batholith at upper crustal conditions an experimental study the understanding of the geochemical and petrophysical evolution of magmas forming intermediate calc alkaline batholiths at shallow crustal levels critically depends on knowledge of the phase equilibria relations along the liquid line of descent here we present experimental results for a tonalitic system at a pressure of 200 mpa and under water saturated conditions melting experiments were performed at temperatures between 700 and 1000 c in externally heated hcm pressure vessels with oxygen fugacity controlled close to the ni nio buffer equilibrium nno employing an argon methane mixture as pressure medium and co pd redox sensors to verify fo2 conditions natural rock powder of a medium k tonalite from the adamello batholith in northern italy served as experimental starting material based on compositional data of stable phases in the run products and images of entire run charges mass balance calculations as well as image processing were performed to investigate the evolution of the crystal melt ratio with respect to temperature furthermore compositional trends of minerals as well as the liquid line of descent of residual melts were obtained orthopyroxene clinopyroxene and plagioclase were identified as near liquidus phases below 990 c at 900 c amphibole joins the solid phase assemblage at the expense of clinopyroxene indicating the existence of a peritectic relationship after an initial near linear decrease with temperature residual melt fractions exhibit a plateau of 45 55 vol between 750 and 850 c followed by a rapid decrease coinciding with quartz saturation at 725 c compositions of residual liquids evolve along a typical calc alkaline differentiation trend with decreasing temperature increasing sio2 and decreasing tio2 al2o3 cao mgo and feo contents and become peraluminous below 900 c intermediate to acidic rocks from the adamello follow the experimental liquid line of descent indicating that the observed compositional spread of the natural intermediate composition rock record can be explained by low pressure magma differentiation and liquid extraction experimentally determined zircon saturation levels are at low temperatures distinctly lower compared to existing and often used zr saturation models but fully consistent with observed and modelled zr evolution trends from the natural rock record of the southern adamello batholith inferring that zircon saturation in these intermediate to felsic plutonic rocks occurred at 800 830 c corresponding to a melt fraction of about 50 vol

the impact of pre existing gas on the ascent of explosively erupted magma most if not all magmas contain gas bubbles at depth before they erupt those bubbles play a crucial role in eruption dynamics by allowing magma to degas which causes the magma to accelerate as it ascends towards the surface there must be a limit to that acceleration however because gas bubbles cannot grow infinitely fast to explore that limit a series of experiments was undertaken to determine the maximum rate at which bubbly high silica rhyolite can decompress rhyolite melt that was hydrated at 150 mpa with 5 3 wt dissolved water and contained 7 to 18 vol bubbles can degas in equilibrium at 875 c when decompressed at rates up to 1 2 mpa s 1 from 150 to 78 mpa and up to 1 8 mpa s 1 when decompressed further to 42 mpa in contrast that same rhyolite cannot degas in equilibrium at 750 c if decompressed faster than 0 015 0 025 mpa s 1 when combined with other published experiments the maximum rate of decompression for equilibrium degassing is found to increase by a factor of ten for every 50 75 c increase in temperature when compared to predictions from conduit flow models that assume equilibrium degassing it is found that such models greatly over estimate the rate at which relatively cold rhyolite can decompress whereas that assumption is largely correct for hot rhyolite and thus for most other magmas all of which are less viscous than rhyolite in addition most bubbles that were 20 30 m in size at high pressure were lost from the population at low pressure that absence suggests that only relatively large vesicles seen in volcanic pumice may be relics of pre eruptive bubbles even if small bubbles were originally present at depth

growth of megaspherulites in a rhyolitic vitrophyre megaspherulites occur in a rhyolitic vitrophyre near the base of a thick rhyolitic vitrophyre that occupies a late eocene to early oligocene volcanic tectonic basin near silver cliff colorado diameters of the megaspherulites range from 0 3 to over 3 66 m including an alteration layer the megaspherulites are compound spherulites they are holocrystalline very fine to fine grained composed of sanidine and quartz and surrounded by a thin sanidine quartz rind and an alteration layer containing mordenite and 15 a montmorillonite megaspherulite crystallization began soon after the vitrophyre was emplaced as the result of sparse heterogeneous nucleation under highly nonequilibrium conditions spherulite growth proceeded in a diffusion controlled manner under lowered viscosity as a result of a high water content 5 to 7 wt which also contributed to sparse nucleation when nucleation did occur it was at a large t 245 350 c on near critical sized nuclei once the spherulites began to grow continued nucleation was on existing growth cones and new independent spherulites did not form sanidine crystals grew with a fibrous habit which is favored by a large t restricting fibril lengths and diameters between 10 30 and 3 7 m respectively during crystallization these growth cones impinged upon each other resulting in fibril cone free areas these cone free interconal areas consist of coarser fine grained phases dominated by quartz which crystallized from the melt as it accumulated between the crystallizing sanidine fibrils of the cones the anhydrous nature of the sanidine and quartz suggests that their crystallization concentrated a water rich residual fluid enriched in ca mg and fe and depleted in si k and na at the megaspherulite vitrophyre interface this fluid phase enclosed the megaspherulites resulting in the hydrothermal alteration of the vitrophyre forming a hydrothermal alteration layer as indicated by the presence of the minerals mordenite and montmorillonite

u th baddeleyite geochronology and its significance to date the emplacement of silica undersaturated magmas baddeleyite is a frequently found accessory mineral in silica undersaturated lavas because it is typically enriched in uranium while having low initial lead baddeleyite has long been a prime target for u pb geochronology of mafic rocks the difficulties in retrieving small baddeleyite grains from volcanic samples and the lack of a detailed understanding of baddeleyite occurrence however have limited baddeleyite chronology largely to coarse grained mafic intrusive rocks here the development of u th in situ baddeleyite analysis using secondary ionization mass spectrometry sims is presented together with an assessment of baddeleyite occurrence in quaternary silica undersaturated lavas from campi flegrei naples italy samples studied comprise the pre and post campanian ignimbrite ca 40 ka lava domes of cuma and punta marmolite and astroni and accademia respectively the in situ sample preparation required initial identification of baddeleyite crystals from sawed and polished rock billets using scanning electron microscope sem backscatter imaging and energy dispersive x ray analysis u th baddeleyite isochron ages for intra caldera astroni and accademia lava domes are 5 01 2 61 2 55 ka mswd 2 0 n 17 and 4 36 1 13 1 12 ka mswd 2 9 n 24 respectively the ages for punta marmolite 62 4 3 9 3 8 ka mswd 1 2 n 11 and cuma 45 9 3 6 3 5 ka mswd 2 2 n 11 predate the campanian ignimbrite rapid baddeleyite crystallization at the time of emplacement is supported by petrologic observations that 50 of the baddeleyite crystals documented in this study occur either in vesicles or in vesicle rich regions of the host lavas whose textures developed over timescales of a few years to a few decades based on microlite crystal size distribution csd analysis radiometric u th baddeleyite ages are mostly in agreement with previously determined k ar eruption ages except for the punta marmolite lava dome whose u th baddeleyite age predates the k ar age by ca 15 ka baddeleyite thus records eruptive emplacement with little evidence for significant pre eruptive crystal residence and has potential as an eruption chronometer for quaternary silica undersaturated volcanic rocks

comagmatic granophyre and dacite from karymsky volcanic center kamchatka experimental constraints for magma storage conditions abstract despite a 30 000 years difference in age two caldera forming eruptions at karymsky volcanic center kamchatka karymsky 7900 yr bp and academy nauk ca 40 000 yr bp produced two pyroxene dacites with the same composition and mineralogy granophyric xenoliths of the identical whole rock chemistry were found in basalts erupted within academy nauk caldera in 1996 unlike the dacites however the granophyres are holo crystalline and contain biotite and amphibole large amphibole phenocrysts contain rare inclusions of clinopyroxenes which compositionally overlap with clinopyroxenes in the dacites the al content of the amphibole suggests it grew at a pressure of about 100 mpa results of hydrothermal experiments and petrologic observations indicate that academy nauk and karymsky dacites were last equilibrated at 883 19 c 100 15 mpa and 871 19 c 85 18 mpa respectively both at water saturated conditions the mineral assemblage of granophyre is reproduced by isobaric crystallization of the dacite at 100 mpa implying that the granophyres were sampled from the crystallized silicic reservoir that produced the caldera forming eruption of academy nauk similar chemical compositions of karymsky and academy nauk dacites indicate that both were derived from the same crustal level source the eruptive history of the calderas can best be explained by two 10 12 km 3 dacitic batches that detached from a parental body situated in the lower crust then ascended to 3 4 km depth re equilibrated and erupted

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

a glimpse into augustine volcano s pleistocene past insight from the petrology of a massive rhyolite deposit abstract activity at augustine volcano alaska has been characterized by intermediate composition domes flows and tephras during the holocene erosive lahars and pyroclastic flows associated with the 2006 eruption revealed large exposures of voluminous rhyolite pumice fall beneath glacial tills the massive rhyolite deposit is evidence of anomalously large prehistoric eruptions the rhyolite is petrologically and mineralogically different from more recent eruptive products with abundant amphibole calcic amphiboles and cummingtonite and quartz three texturally and chemically distinct lithologies are present fe ti oxide equilibria suggest temperatures of 765 c and oxygen fugacities of nno 1 5 melt inclusions indicate that magma representing the stratigraphically lowest lithology was crystallizing at 260 mpa with a contemporary mixed h2o co2 fluid phase becoming progressively more h2o rich magma forming the other lithologies crystallized in the presence of this h2o dominated fluid as demonstrated by the presence of cummingtonite and little to no co2 in melt inclusions amphibole and quartz feldspar melt equilibria models yield results indicating a range of crystallization pressures from 400 mpa to 175 mpa apatites and melt inclusions have lower chlorine contents than more recently erupted material at augustine suggesting that the composition of augustine s volatile phase has changed over time reheating textures in melt inclusions and phenocrysts as well as the presence of xenocrysts of olivine and clinopyroxene containing mafic melt inclusions signify the likelihood of mixing and or mingling of magmas as an eruption trigger the unique qualities of this pleistocene rhyolite and the potential hazard of a similarly large eruption in modern times indicate that further study is warranted

oh bearing planar defects in olivine produced by the breakdown of ti rich humite minerals from dabie shan china the partial breakdown of ti chondrodite and ti clinohumite during exhumation from ultra high pressure to amphibolite facies conditions in garnet pyroxenites from dabie shan china produces coronas of olivine coexisting with ilmenite blebs fourier transform infrared ftir spectra of this newly formed olivine exhibit absorption bands in the hydroxyl stretching region two intense peaks were observed at 3 564 and 3 394 cm 1 identical in energy to peaks in ti clinohumite transmission electron microscopy tem of the same olivine domains revealed the presence of a complex 001 planar intergrowth these interlayers have a 1 35 nm repeat distance which is characteristic of clinohumite such interlayers are also enriched in ti with respect to the adjacent olivine as shown by energy dispersive spectrometry the combined evidence from ftir spectroscopy and tem indicates that oh is incorporated along ti clinohumite planar defects this study provides evidence that the nominally anhydrous phase olivine may contain oh as a humite type defect beyond the breakdown of the hydrous humite minerals and confirms earlier suggestions that ti plays a key role in oh incorporation in mantle olivine we suggest that olivine containing ti clinohumite defects is an important phase for water transport in subduction zones and for the storage of water in cold subcontinental mantle however these defects are unlikely to be stable in hotter parts of the oceanic mantle such as where basaltic magmas are generated

the genesis of arc dacites the case of mount st helens wa throughout the last 35 ka mount st helens has been the most active volcano in the cascade arc but the origin of its voluminous dacites remains controversial these dacites were traditionally interpreted as a result of melting metabasaltic lower crust yet recent studies have challenged this view and suggested an origin dominated by differentiation of mafic magmas through assimilation fractional crystallization afc processes to address this discrepancy on the origin of dacites at mount st helens we conduct an interdisciplinary study using a combination of thermal and geochemical modeling our results show that 45 crystallization of a basaltic andesite parent reproduces the compositions of the dacites with a maximum of 20 30 assimilation of lower crustal lithologies amphibole textures and compositions support such a differentiation trend in a polybaric mush system combined with recent geophysical imaging and experimental data we suggest that mount st helens dacites are generated by 1 mantle derived arc magma evolving by afc to intermediate compositions in a lower crustal magma reservoir and 2 ascent of these magmas to a mid to upper crustal reservoir where they reach high crystallinity without significant further chemical differentiation and are subject to frequent recharge that leave a clear mixing mingling overprint

cummingtonite and the evolution of the mount st helens washington magma system an experimental study hydrothermal experiments have been conducted on the 3 5 ka cummingtonite bearing yn tephra from mount st helens washington to determine the stability field of cummingtonite in volcanic rocks and to gather additional data on the evolutionary history of the mount st helens magmatic system cummingtonite is not stable in experiments conducted at or above 810 c at pressures 2 o analyses of melt inclusions that yield preeruption melt h 2 o contents of 6 4 wt the yn tephra belongs to a series of cummingtonite bearing dacites that erupted early in the history of mount st helens these early dacites have relatively homogeneous compositions and are characterized by low temperature 800 c high water pressure and high oxygen fugacity nno 1 phenocryst crystallization silicic magmas that erupted after eruption of basaltic andesite at mount st helens 2 3 ka are more heterogeneous in composition and last equilibrated under higher temperatures 900 c lower water pressures and lower oxygen fugacities than the early dacites these changes in the magma chamber conditions appear to have been related to the intrusion of mafic magma into an upper crustal dacitic magma chamber

phase equilibria constraints on pre eruptive magma storage conditions for the 1956 eruption of bezymianny volcano kamchatka russia abstract phase equilibria experiments were performed on andesites from the catastrophic 1956 eruption of bezymianny volcano kamchatka russia to determine pre eruptive magma storage conditions fifteen experiments were conducted under water saturated conditions with oxygen fugacity equal to the ni nio oxygen buffer at temperatures between 775 and 1100 c and pressures between 50 and 200 mpa simultaneous amphibole and plagioclase crystallization is reproduced at 850 c and 200 mpa the simultaneous crystallization temperature range of the plagioclase clinopyroxene orthopyroxene fe ti oxide assemblage increases with decreasing pressure from 840 to 940 c at 150 mpa to 940 1020 c at 50 mpa melt inclusion compositions in plagioclase phenocrysts and matrix glass match experimental melt compositions reproduced at 50 100 mpa and 50 mpa respectively presence of the silica phase in groundmass and mature amphibole breakdown rims suggests that magma has been stored at ca 3 km depth prior to the final ascent for at least 40 days syn eruptive ascent led to decompression driven crystallization which caused a temperature increase from 850 900 c to 950 1000 c

the influence of magma ascent path on the texture mineralogy and formation of hornblende reaction rims abstract although hornblende reaction rims are widely used as a tool for evaluating magma ascent during volcanic eruptions very few studies constrain the manner in which they form this study investigates the influence of magma ascent path on the formation of hornblende reaction rims to do this we conducted isothermal 840 c decompression experiments using dacite pumice samples erupted in december 1989 from redoubt volcano alaska experiments were first held within the hornblende stability field determined through phase equilibria experiments for 3 to 5 days before being decompressed to different pressures ranging from 100 to 2 mpa for 1 to 30 days before being quenched decompression was performed in either multiple equal steps constant rate or in one single step results from multi step experiments show that reaction rims form preferentially at pressures from 10 to 40 mpa and that this favorable pressure range narrows and decreases with increased decompression duration hornblendes in multi step experiments are tightly enclosed by fine grained reaction rims composed of plagioclase and orthopyroxene crystals with high aspect ratios ranging from 1 to 12 for single step experiments reaction rims also form preferentially within a narrow pressure range 60 70 mpa where they consist of medium grained plagioclase titanomagnetite and orthopyroxene crystals with aspect ratios ranging from 1 to 6 that broadly surround subrounded hornblendes hornblendes from single step experiments dropped to lower pressures

monitoring the 1980 1982 eruptions of mount st helens compositions and abundances of glass the mount st helens eruptive sequence of 1980 through 1982 reflects the tapping of successively less water rich more highly crystallized and more viscous highly phyric dacitic magmas these changes reflect both syn and preeruption processes the decreasing water content points to a continued decline in the volume and intensity of explosive pyroclastic activity this decreasing water content appears to be composed of a long term trend established during a long period of repose about 130 years imposed on short term trends established during short periods about 7 to 100 days of repose between eruptions in the present eruptive cycle the last two eruptive cycles of this volcano the t a d 1800 and w cycles about a d 1500 exhibited similar trends these changes are inferred from a combination of petrographic bulk chemical and electron and ion microprobe analyses of matrix and melt inclusion glasses

the acoculco caldera complex magmas genesis evolution and relation with the acoculco geothermal system abstract the acoculco caldera complex acc is located in the eastern part of the trans mexican volcanic belt mexico the acc was formed 2 7 ma and since then volcanic activity has persisted until 0 06 ma through the emission of domes cinder cones fissure lava flows and two ignimbrite eruptions dated at 1 2 and 0 65 ma the pervasive hydrothermal alteration in the central part of the acc has motivated considerable geothermal exploration however none of it assessed the relation between the magmatic heat supply and the evolution of magmas including their reservoir depth magmatic processes were investigated with petrography major oxides trace element geochemistry mineral chemistry and isotopic analysis in addition we performed a series of hydrothermal experiments in order to constrain the storage depth for the magma tapped 1 2 ma during a reactivation of the caldera we investigated the origin of magmas tapped during the eruptive history of the acc and the magmatic processes magma mixing assimilation and crystallization that modified these magmas after the caldera collapse 2 7 ma the local stress field was probably modified and allowed the ascent of peralkaline magmas through new plumbing systems such magmas mixed with calc alkaline magmas and formed the post caldera volcanism influx of new magma is considered as the probable heat supply that maintains the active magmatic system whereas assimilation has been negligible but yields insights about the depth at which processes have occurred our work suggests that there are either shallow storage zones 200 500 bars where magmas crystallize and either erupt or stall and cool or there are intrusive magmas unrelated to the acc suite that have accumulated nearby and heat the reservoir magmas

geothermometry and geobarometry of a cummingtonite bearing dacite from martinique lesser antilles a dacite sample from a nuee ardente deposit at pitons du carbet martinique contained cummingtonite orthopyroxene quartz magnetite and ilmenite as phenocrysts and thus was suitable for evaluation of the t f text o text 2 p text total p text h text 2 text o conditions of crystallization of nuee ardente materials according to the method of ewart et al 1971 the estimations were obtained with the aid of a fortran iv program which permitted the operation of all the arrangements between the available microprobe analyses the results indicated that the phenocrystic association crystallized at moderate temperatures ca 750 c under high oxygen fugacities nno and total pressures 2 3 4 5 kb with a maximal density of points at about 3 5 kb and water pressures approximating total pressures the groundmass fe ti oxides equilibrated at lower temperatures and under relatively increasing oxygen fugacities the variation of which is tentatively ascribed to the aerial emplacement of the nuee ardente materials the phenocryst data imply that in the case studied the high fluid pressures characterizing the nuee ardente eruptions were not acquired at a shallow level because of the deep intratelluric stage of crystallization of these differentiated calcalkalic magmas

mineralogy and phase chemistry of mount st helens tephra sets w and y as keys to their identification abstract voluminous and widespread tephras were produced frequently during the last 36 000 yr of volcanic activity at mount st helens numerous tephra sets have been defined by d r mullineaux j h hyde and m rubin 1975 u s geological survey journal of research 3 329 335 on the basis of field relations fe mg phenocryst assemblage and 14 c chronology and are valuable marker beds for regional stratigraphic studies in this study modal abundances and mineral compositions were determined via petrographic and electron microprobe techniques for numerous samples of individual layers within tephra sets w and y to evaluate the degree of compositional variability within and between tephra layers and criteria by which to distinguish among mount st helens and other pacific northwest tephras although individual layers within a set e g we wn cannot be distinguished from each other on the basis of mineralogic characteristics examined mineral compositions allow distinction among layers w and y and other pacific northwest tephras e g mazama glacier peak fe ti oxide compositions and t f o 2 estimates derived using coexisting magnetite ilmenite are especially useful due to the compositional homogeneity of these minerals both within and between samples of a given unit over a wide geographic area the silicates show more compositional variability than the oxides but io 2 al 2 o 3 contents in hornblende and fe mg ratios in hypersthene aid in distinguishing among pacific northwest tephras

summary of pre 1980 tephra fall deposits erupted from mount st helens washington state usa mount st helens has been a prolific source of tephra fall deposits for about 40 000 years these tephra deposits 1 record numerous explosive eruptions 2 form important regional time stratigraphic marker beds and 3 record repeated changes in composition within and between eruptive periods

high ti amphibole as a petrogenetic indicator of magma chemistry evidence for mildly alkalic hybrid melts during evolution of variscan basic ultrabasic magmatism of central iberia central iberian variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous high k calc alkaline ultrabasic to intermediate rock series despite their overall calc alkaline affinity the mafic ultramafic members contain high ti amphibole oikocrysts rimmed by lower ti amphibole cummingtonite and high ti amphibole replacing early phlogopite to understand the factors controlling the saturation of high ti amphibole in the parental magmas clinopyroxene melt phlogopite melt and amphibole melt relationships are reviewed this analysis reveals that for melts with intermediate compositions the affinity of tio2 for amphibole rises in alkalic magmas accordingly mildly alkalic trachytoid to subalkaline medium to high k andesite and dacite compositions are estimated for interstitial high ti amphibole saturated melts amphibole ce pb ratios reveal a mantle crust hybrid nature for interstitial melts with subalkaline trachytoid compositions the hydrous character of the variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of na2o with respect to silica during amphibole crystallization

heterogeneous bubble nucleation on fe ti oxide crystals in high silica rhyolitic melts the nucleation of h2o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 740 800 c and 50 175 mpa decompressed by 20 70 mpa and held at the lower pressures for 10 s before being quenched the hydration conditions were subliquidus and all samples contain blocky magnetite needle shaped hematite plagioclase magnetite is abundant at 800 c and high pressures whereas hematite becomes more abundant at lower temperatures and pressures bubbles nucleated in a single event in all samples with the number density nt of bubbles varying between 2 107 and 1 109 cm 3 at low degrees of supersaturation one to a few bubbles nucleate on faces of magnetite but at medium to high degrees of supersaturation multiple bubbles nucleate on single magnetite grains on hematite one to a few bubbles nucleated at the ends of the needle shaped crystals at medium supersaturations but formed along their entire lengths at high supersaturations nt increases as water diffusivity decreases indicating that the number of bubbles nucleated is influenced by their growth which depletes the melt with respect to h2o and lowers supersaturation if volcanic eruptions are triggered by bubble formation in magmas stored in shallow level magma chambers then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed whereas homogeneous nucleation is unlikely to trigger eruptions

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab