integrating the uranium series and elemental diffusion geochronometers in mixed magmas from volcan quizapu central chile magma mixing is a ubiquitous process in silicic magmatic systems we present pre and post mixing time scales for a single magma mixing event associated with the 1846 1847 eruption of volcan quizapu we compare and contrast 226 ra 230 th crystal residence ages between the mixed lavas of the 1846 1847 effusive eruption and the largely homogeneous plinian dacite eruption of 1932 both eruptions are dominated by the same dacite magma whereas the ear lier eruption contains evidence for a single episode of mafic recharge and mixing the use of uranium series mineral separate dating on a mixed magma requires deconvolution of different crystal populations we present a four component mixing model for the crystal separates to obtain dacite derived plagioclase model ages as well as andesite derived recharge plagioclase ages plagioclase model ages average 3 6 ka for dacite derived plagioclase from the mixed 1846 1847 eruption 1 4 kyr older than average plagioclase model ages of the 1932 eruption suggesting that magma mixing and reheating led to some resorption of young crystal rims in the 1846 1847 magmatic episode and or that some growth of plagioclase rims occurred during the repose time of 86 years the andesite derived plagioclase residence ages are younger than the corresponding dacite derived plagioclase residence ages but they are still of the order of thousands of years in addition we use mg diffu sion in plagioclase to constrain time scales of magma mixing we show that the 1846 1847 eruption was triggered by large volume mafic recharge magma that mixed and mingled with the resident dacite within probably days to weeks of the ensuing eruption

dynamic deformation of seguam island aleutian islands alaska 1993 2000 implications for magmatic and hydrothermal processes 1 the most recent volcanic eruption on seguam island was detected in december 1992 and lasted until july or august of 1993 a series of radar interferograms generated from imagery collected by the european space agency s ers 1 and ers 2 satellites are used here to map the spatial and temporal evolution of the deformation field from june of 1993 to september of 2000 this evolution consists of a coeruptive period of uplift surrounding the eruption two posteruptive subsidence signals centered within each of the two calderas on the island each of which lasted for 4 years after the end of the eruption and then uplift beneath the caldera on the eastern side of the island and the surrounding region from mid 1999 through the end of the time series in september 2000 the locations and magnitudes of the sources of the deformation are inferred from the interferometric synthetic aperture radar displacement data using an elastic half space earth model with multiple mogi point sources and a levenburg marquardt nonlinear inversion technique this inference reveals that the sources of subsidence are located within the upper 2 km of the earth s crust while the sources of uplift are located below 2 5 km the locations of dilatational sources are consistent with a model of the volcanic system where ascending magma accumulates below a density or structural boundary near 2 5 3 km depth the subsidence signals are most likely associated with cooling of shallow magmatic bodies emplaced during the 1992 1993 eruption however the possibility that they indicate a combination of magma cooling and depressurization of a hydrothermal system cannot be ruled out using the geodetic data alone implications for the long term evolution of the volcanic edifice are discussed

one hundred million years of geomagnetic polarity history since 1968 absolute ages have been assigned to the late cretaceous cenozoic geomagnetic reversal time scale by fixing the ages of two or more calibration points in a composite marine magnetic anomaly profile and interpolating between or extrapolating beyond these points assuming constant spreading rates in each interval previously no more than 4 calibration points were used but it is now possible to specify 11 calibration points in addition to the 0 m y datum this improvement is based on magnetostratigraphic studies in italian pelagic limestones these studies closely tie the geomagnetic reversal sequence to the foraminiferal and coccolith zonations absolute ages of calibration points are provided by the best available dates on stage boundaries which are located from the biostratigraphic zonation the greatest changes from previous scales come in the late paleocene early eocene where the new ages are as much as 3 m y younger than in the 1977 scale of labrecque and others and as much as 1 8 m y younger than in the 1980 scale of ness and others

magmatic evolution of a volcano studied by 230th 238u disequilibrium and trace elements systematics the etna case age determinations of several lava flows from etna through 230th 238u disequilibrium internal isochrons yield a precise chronology of the volcano s history for the last 200 000 years and emphasize the main episodes in the formation of this huge complex strato volcano study of 230th232th 0 initial ratios of lavas together with their trace element compositions yields a consistent model of magmatic evolution implying the existence for 200 000 years of a deep reservoir of alkalic magma periodically mixed with magmas of tholeiitic affinity these short periods of mixing appear to be related to the formation of the large calderas of etna in addition to these processes affecting the deep reservoir fractional crystallization also occurred in more superficial levels of the volcanic edifice thereby yielding several series of differentiation of relatively short duration as for its geochemistry etna s volcanism is of oceanic type but with its own characteristics and in a peculiar geodynamic context at the edge of the african plate

magnetic susceptibility anisotropy strain and progressive deformation in permian sediments from the maritime alps france abstract the relationships among magnetic susceptibility anisotropy finite strain and progressive deformation have been studied in permian red shales and slates of the maritime alps southeastern france these rocks contain deformed reduction spots which serve as finite strain indicators the magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements the magnetic fabric changes progressively from oblate to prolate and back to oblate as deformation increases the quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined they differ between the less and more deformed areas perhaps due to a change in deformation mechanism accompanying an increase in metamorphism the relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control

transition from extensional to compressional magnetic fabrics in the cretaceous cabuerniga basin north spain abstract the transition between extensional and compressional driven magnetic fabrics in sedimentary rocks is explored in this paper through the study of an example of the basque cantabrian basin in the area where extensional structures prevail and no superimposed deformation is observed except for gentle large scale folds the magnetic fabric is interpreted as extensional in consistency with mesostructural tension gashes and macrostructural large scale faults data compressional tectonic fabrics are unequivocally interpreted in the area with cleavage development related to the buttressing of the syn rift sequence against faults located near the northern basin margin in this area k max is oriented according either to the intersection lineation or the dip direction of cleavage planes in the area located in between where no macroscopic evidence of either compression or extension exist there is a transitional fabric between compressional resulting from the modification during inversion of a previous sedimentary or extensional fabric and extensional inherited from the extensional stage magnetic fabrics that correlate with subtle evidences at the microscopic scale pressure shadows deformation and re orientation of nodules therefore the magnetic fabric is revealed as an exceptionally sensitive marker of deformation in sedimentary rocks this transition in the magnetic fabric occurs within a length of 6 25 km along the cross section that correlates with a thickness of 200 m of the stratigraphic pile these results indicate that even in the absence of clear structural markers of compressional deformation extensional magnetic fabrics can be only interpreted when there is a minimum separation in the vertical or the horizontal to the cleavage front

discriminating assimilants and decoupling deep vs shallow level crystal records at mount adams using 238u 230th disequilibria and os isotopes article a suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the mount adams volcanic field has been analyzed for u th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis all of the lavas have 230 th 238 u n1 and span a large range in 230 th 232 th ratios

volcanic history and magmatic evolution of seguam island aleutian island arc alaska new 40 ar 39 ar dating coupled with detailed field mapping stratigraphy and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of seguam island in the aleutian island arc alaska sixty new 40 ar 39 ar ages from lavas domes and pyroclastic deposits were obtained using furnace incremental heating techniques on replicate samples of whole rock and ground mass separates and they constrain the duration of pleistocene to holocene subaerial volcanism to 318 k y the 40 ar 39 ar plateau ages indicate that over 85 of the complex 68 km 3 of material was erupted almost continuously between 318 ka and 9 ka at ca 9 ka a stratocone on the eastern half of the island partially collapsed producing a 4 km wide crater rhyolitic dome forming eruptions followed from vents in the newly created crater and were likely contemporaneous with 8 0 km 3 of basaltic and basaltic andesitic effusions from pyre peak and a 1 4 km 3 basaltic eruption from a monogenetic cone on the far eastern end of the island geochemical changes over the last 318 k y are subtle most notably the earliest eruptions from 318 to 142 ka produced no andesite and basalt from this period has larger ranges in zr rb and la yb than younger basalts small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal liquid fractionation process that varied only slightly in successive eruptive phases over 318 k y we identified minor geochemical changes in magma composition during each of the three main stages of volcanism but overall the monotonic variations in major and trace element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed system fractional crystallization of basalt using the 40 ar 39 ar geochronology and estimates of individual flow volumes we calculated a time averaged eruptive rate at seguam that is similar to growth rates of other well dated arc volcanoes in the cascades and chilean southern volcanic zone but less than that of mount katmai and mount mageik which are located on the alaska peninsula the eruptive flux at seguam has been highly variable fluctuating more than an order of magnitude from 0 07 km 3 k y during the early history of bimodal volcanism to 1 18 km 3 k y over the past 9 k y

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

magnetic fabric investigation in the northwestern sichuan basin and its regional inference abstract an investigation of the anisotropy of magnetic susceptibility ams in 39 sites has been carried out in the northwestern sichuan basin which is adjacent to the northeastern tibetan plateau discussing the deformation pattern of the basin and the orogenic process of the corresponding orogens stepwise demagnetization of three orthogonal isothermal remanent magnetization suggests that hematite is the main magnetic carrier mineral combined with the low value of magnetic susceptibility it is considered that the ams is some combination of hematite and paramagnetic matrix minerals different types of magnetic fabrics from sedimentary to pencil structure magnetic fabric and sharps of susceptibility ellipsoids from oblate to prolate were observed the existence of sedimentary magnetic fabric and the accordance of magnetic lineation and bedding strike indicate the magnetic fabrics along the front of longmen shan belt were the result of tectonic deformation thermochronology researches in the orogens and our apatite fission track analysis in the northwestern sichuan basin suggest that magnetic fabric mainly reflects the cenozoic deformation the magnetic lineation which is consistent with the fault and fold strikes in the longmen shan was induced by mesozoic and cenozoic deformation and the latter is the primary one besides the typical weak deformation magnetic fabrics atypical magnetic fabrics characterized by a magnetic lineation that is oblique to the bedding strike are shown in 53 sites along the front of micang shan it is proposed that such a type of atypical magnetic fabric is the result of two competing sub fabrics in the weakly deformed tectonic superposition region

shallow and deep crustal control on differentiation of calc alkaline and tholeiitic magma article u 230 th isotopes 87 sr 86 sr lower crust assimilation the role of changing crustal interaction and plumbing geometry in modulating calc alkaline vs tholeiitic magma affinity is well illustrated by the influence of 70 km thick crust beneath volcan parinacota changes in petrologic affinity correlate with periods of cone building sector collapse and rebuilding of the volcano over the last 52 ka and are well explained by changes in magma recharge regime with increasing recharge and magma output lavas transition from low fe strongly calc alkaline phenocryst rich silicic compositions to medium fe near tholeiitic mafic and aphanitic characteristics strontium isotope data show that the change in magma regime did not affect all parts of the system simultaneously these are characterized by distinctive 87 sr 86 sr ratios which suggest an initially compartmentalized system relatively high 230 th 232 th activity ratios of 0 72 in early erupted calc alkaline lavas are consistent with interaction with high u upper crust low 230 th 232 th activity ratios of 0 55 and up to 33 th excess in younger near tholeiitic lavas correlate with steep ree patterns indicating lower crustal interaction thorium excesses at the time of eruption approach the maximum that can be generated via small degree garnet residual melting in the lower crust or mantle and imply that transit time through the crustal column for the most tholeiitic magmas had to be short on the order of b2 10 4 yr in contrast lavas with greatest calc alkalinity are also at or near secular

volcanism and erosion during the past 930 k y at the tatara san pedro complex chilean andes geologic mapping together with 73 new k ar and 40ar 39ar age determinations of 45 samples from 17 different volcanic units plus paleomagnetic orientations geochemical compositions and terrestrial photogrammetry are used to define the chronostratigraphy of the tatara san pedro complex an eruptive center at 36 s on the volcanic front of the andean southern volcanic zone the tatara san pedro complex preserves 55 km3 of lavas that erupted from at least three central vent regions remnant unconformity bound sequences of lavas are separated by lacunae that include significant periods of erosion quaternary volcanism commenced ca 930 ka with eruption of voluminous dacitic magma followed 100 k y later by the only major rhyolitic eruption from 780 ka onward more than 80 of the preserved volume is basaltic andesite 52 57 sio2 but petrographically and geochemically diverse dacitic magmas 63 69 sio2 erupted sporadically throughout this younger dominantly mafic phase of activity a few basaltic lavas 49 52 sio2 are present mainly in portions of the complex older than 230 ka the number of vents the petrologic and geochemical diversity and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the tatara san pedro complex over the past million years nearly two thirds of the preserved volume of the tatara san pedro complex comprises the two youngest volcanoes which were active between ca 188 83 ka and 90 19 ka repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre 200 ka volcanic record particularly on the south flank of the complex dating the inception of a glaciation on the basis of preserved material is difficult but the age of the oldest lava above a lacuna may be used to estimate the timing of deglaciation on this basis the argon ages of basal lavas of multiple sequences indicate minimum upper limits of lacunae at ca 830 790 610 400 330 230 110 and 17 ka these are broadly consistent with global ice volume peaks predicted by the oxygen isotope based astronomical time scale and with age brackets on north american glacial advances estimated growth rates for the two young volcanoes are 0 2 to 0 3 km3 k y these are three to five times greater than a growth rate estimated from all preserved lavas in the complex 0 06 km3 k y removal of up to 50 95 of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy and it raises the possibility that episodic erosion of mid latitude frontal arc complexes may be extensive and common

holocene tephra succession of puyehue cordon caulle and antillanca casablanca volcanic complexes southern andes 40 41 s abstract puyehue cordon caulle and antillanca volcanic complexes are two of at least 50 active frontal arc volcanoes that define the 1400 km long southern volcanic zone of chile holocene tephra deposits in chile and argentina 40 41 s up to 100 km east of these volcanoes comprise at least five voluminous 1 to 8 km 3 pyroclastic fall layers that preceded several recently deposited cordon caulle pumice fallouts field observations of proximal medium and distal facies of the deposits in conjunction with geochronology and geochemistry of the volcanic complexes indicate that three fall layers are derived from puyehue volcano puyehue 1 and 2 and mil hojas whereas two are sourced from the antillanca complex playas blanca negra and nahuel huapi tephra 20 km to the south the oldest tephra calibrated 14 c age 10 49 0 12 ka 2 found only at medium distal facies is deposited directly on granitic moraine boulders and consists of deeply weathered orange dacitic pumice lapilli the next prominent tephra at 7 ka comprises dacitic pumice and its age is equivalent to a rhyodacitic dome exposed in the puyehue summit crater above these deposits there are phases of a complex eruption consisting of a conspicuous compositionally zoned tephra it also comprises a pyroclastic density current together with lithic rich and scoriaceous fallout deposits mineralogical geochemical and sr isotope evidence plus the isopach maps confirm that this sequence of eruptive events is sourced from antillanca at 1932 68 yrbp the total volume of this eruptive sequence exceeds 8 km 3 making it the largest holocene eruption from either volcanic complex this eruption was likely responsible for the destruction of an ancestral antillanca volcano and the formation of a 4 5 km diameter caldera a distinctive younger unit in the region is a voluminous rhyodacitic pumice fall calibrated 14 c age 1 11 0 07 ka above which a series of several alternating dark lithic and pumice lapilli beds accumulated correlation with proximal deposits indicates that the 1 11 ka eruption was derived from puyehue and destroyed 3 km 3 of rhyodacitic domes at this volcano summit historic explosive activity at the nascent casablanca volcano and along cordon caulle including the 2011 2012 eruption 1 km 3 of uncompacted pumice the largest from this fissural zone emphasizes an increased risk for volcanic hazards in central chile and argentina

reply to comment on steady state 226ra 230th disequilibrium in mantle minerals implications for melt transport rates in island arcs by r george m reagan s turner j gill b bourdon abstract we agree with george et al that high 226 ra 230 th and correlated ba th 238 u 230 th and other elemental and isotopic ratios are the result of fluid addition to the wedge the feineman and depaolo earth planet sci lett 215 2003 339 355 manuscript addresses more specifically the pathway followed by the fluid en route to the surface in particular we address whether the 226 ra 230 th data directly yield the total time available for the fluid to migrate from the point of origin in the slab to the melting region in the wedge and ultimately to the surface with the melt all of the processes involved in generating arc volcanism would of necessity have to take place very rapidly within the constraints proposed by george et al the alternative is that the upward fluid movement is hindered by the fluid flow regime and solid mantle flow allowing more time for reaction with the mantle wedge there is evidence in the u series data for more than one time scale and our model helps to explain how this could come about the extent to which grainscale 226 ra 230 th disequilibrium affects the inferred melt transport time is yet another issue and this could be achieved either with the small degree melt mechanism we discussed or by additional effects associated with melt and fluid migration at later stages in the melt generation process we agree that the final melt transport event is likely to happen quickly one of the most puzzling features of volcanic arcs is that the volcanic front emerges considerably behind the presumed location of primary water release from the subducting slab depth to slab is 120 km beneath the volcanic front as opposed to the predicted 80 km this offset between the expected site of fluid release and the location of the volcanic front can be attributed to coupled transport of the fluid in the form of hydrous minerals such as amphibole and or phlogopite in the wedge j geophys res 97 1992 2037 2070 or redirection of the melt due to flow and stresses in the mantle matrix as the magma slowly percolates upward j geol 95 1987 285 307 in either case the fluid and or melt is required to ascend slowly at least for some part of its journey to explain the location of the volcanic front furthermore 238 u 230 th disequilibria in arc lavas seem to suggest a relatively long interval for fluid addition 30 60 ka science 292 2001 1363 1366 earth planet sci lett 179 2000 581 593 geochim cosmochim acta 61 1997 4855 4884 which is difficult to reconcile with the 226 ra 230 th disequilibrium observed in the lavas for these reasons we sought to find a means whereby apparently short lived isotopic disequilibria could be maintained during complex fluid and melt migration in the mantle wedge below we address a few of the main issues presented by george et al earth planet sci lett this issue in their comment

rapid magma ascent and generation of 230 th excesses in the lower crust at puyehue cordon caulle southern volcanic zone chile basaltic to rhyolitic lavas and tephras erupted over the last 70 kyr at the puyehue cordon caulle volcanic complex in the andean southern volcanic zone svz were analyzed for major and trace element sr isotope and u th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis internal u th mineral isochrons from five lavas and three tephra fall deposits are indistinguishable from their eruption ages indicating a short period b1000 yr of crystal residence in the magma prior to eruption the 230 th 232 th ratios define a narrow range 0 80 0 83 compared to that of all svz lavas 0 72 0 97 suggesting that puyehue basalt was derived from a relatively uniform mantle source dacites and rhyolites have the largest u excesses and likely evolved via fractional crystallization of a plagioclase dominated mineral assemblage in contrast basalts have 1 to 6 230 th excesses a characteristic not previously observed in frontal arc stratovolcanoes of the andean svz the 230 th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust lower crustal processes therefore have dampened the 238 u excesses that were generated during fluid addition to the mantle wedge although prior 238 u 230 th 226 ra studies of lavas from other southern svz stratovolcanoes 36 to 41 s have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc our results suggest that mash processes envisioned by hildreth and moorbath w hildreth s moorbath crustal contributions to arc magmatism in the andes of central chile contrib mineral petrol 98 1988 455 489 in the northern svz also occur in the southern svz where the crust is relatively thin

on the age of the laschamp geomagnetic excursion abstract the laschamp geomagnetic excursion is a critical tie point found directly in deep sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice and to providing an independent calibration of the radiocarbon calendar its timing remains poorly known previous k ar 40 ar 39 ar and u th isochron determinations from lava flows at the type locality in the massif central france vary widely are imprecise and suggest a mean age of about 46 2 2 5 ka 2 results of 6 new unspiked k ar and 13 40 ar 39 ar incremental heating experiments on subsamples from three sites on the laschamp and olby flows are concordant and give a weighted mean age of 40 4 1 1 ka 2 uncertainty including analytical sources only that is 10 younger than the previous estimates considering that the 40 k 40 ar decay constant is not known to a precision better than 2 4 the most probable radioisotopic age for the laschamp excursion is 40 4 2 0 ka 2 analytical plus decay constant uncertainties this new age for the laschamp excursion agrees precisely with that deduced from the napis 75 deep sea sediment paleointensity stack when calibrated against the gisp2 ice core chronology using the o isotopes in ice and the magnetic properties of the marine cores

crustal differentiation processes at krakatau volcano indonesia anak krakatau is a basaltic andesite cone that has grown following the famous caldera forming 1883 eruption of krakatau it breached sea level in 1927 and since the 1950s has been growing at an average rate of similar to 8 cm a week we present new major and trace element data combined with whole rock delta o 18 sr and nd isotope data for 1883 1993 and 2002 krakatau eruptive products and the surrounding crust bombs erupted from anak krakatau during 2002 contain frothy metasedimentary and plutonic xenoliths that show variable degrees of thermal metamorphism plastic deformation and partial melting contact metamorphic minerals such as cordierite and tridymite in metasedimentary xenoliths are consistent with high temperature metamorphism and incorporation at mid to upper crustal depth energy constrained assimilation and fractional crystallization modelling of whole rock data suggests that the anak krakatau magmas have a genetic relationship with the 1883 eruption products the geochemical impact of crustal contaminants on whole rock compositions is apparently small and we conclude that low levels of assimilation of a quartzo feldspathic sediment are recorded in anak krakatau magmas plagioclase phenocrysts from the 2002 eruption exhibit disequilibrium textures and complex compositional zoning however and are also isotopically variable with a total range in sr 87 sr 86 of 0 center dot 7043 0 center dot 7048 as determined by in situ laser ablation inductively coupled plasma mass spectrometry this suggests that although shallow crustal assimilation appears to have had a limited effect on whole rock chemistry a complex late stage differentiation history is recorded within the magma s cargo of crystals and xenoliths

extensional tectonics on sardinia italy insights into the arc back arc transitional regime although the tectonic features and stress regime typical for accretionary complexes and back arc domains have been widely documented so far few are known on the transitional zone separating these two systems here we report on structural analysis and anisotropy of magnetic susceptibility ams results from eocene pliocene sediments exposed in western sardinia from late oligocene to middle miocene the studied area was located between the alpine apennine wedge to the east which was undergoing shortening and accretion and the liguro provencal basin undergoing extension and spreading we find that prior to the formation of the liguro provencal basin the middle eocene lower oligocene sediments cropping out at the southwesternmost edge of sardinia were subjected to ne sw shortening in present day coordinates in agreement with recently reported geological information conversely the upper oligocene pliocene sedimentary sequences record a different evolutionary stage of extensional processes upper oligocene middle upper burdigalian sediments clearly show a n s oriented magnetic lineation that can be related to extensional direction along the prevalent e w oriented normal faults on the other hand no magnetic lineation has been detected in upper burdigalian serravallian sediments which mark the end of the first rifting process in sardinia which likely coincides with the rift to drift transition at the core of the liguro provencal basin finally a ne sw extension is observed in two tortonian pliocene sites at the northwestern margin of the nnw sse oriented campidano graben our study confirms that ams may represent a valuable strain trajectory proxy and significantly help to unravel the characters of temporally superimposed tectonic events

distinct magnetic fabric in weakly deformed sediments from extensional basins and fold and thrust structures in the northern apennine orogenic belt italy we report on results from anisotropy of magnetic susceptibility ams analyses carried out on weakly deformed fine grained sediments from the northern apennine orogenic system italy we sampled 63 sites from preorogenic synorogenic and postorogenic sequences which differ in age composition depositional environment degrees of deformation and tectonic regimes the magnetic fabric is typical of weakly deformed sediments with a magnetic foliation subparallel to the bedding plane and a magnetic lineation well defined in this plane northern apennine chain deposits are characterized by strongly oblate magnetic susceptibility ellipsoids indicating that the magnetic fabric is the result of both compaction process and tectonic load experienced by the sediments during diagenesis and orogenic events the orientation of magnetic lineation is significantly different depending whether the studied sites underwent extensional or compressional tectonic regimes in the northern apennine chain the magnetic lineation is mostly oriented nnw sse parallel to the main compressional structures it suggests a tectonic origin of the magnetic lineation with an acquisition related to the apennines compressional phases in the extensional tuscan tyrrhenian margin magnetic lineation is oriented ene wsw almost perpendicular to the main extensional faults which represent the main deformation elements of the area our results demonstrate a close relationship between the shape and orientation of magnetic fabric and the tectonic history of rocks confirming that ams represents a valuable tool to investigate the tectonic history of weakly deformed sedimentary rocks

late folding related magnetic foliation in the active ferdows northeastern iran thrust fold system abstract in this work we present new ams and structural results from the ferdows active fold thrust system lut block central iran ams analyses show that the miocene units of the upper red formation have a well defined magnetic fabric of tectonic origin with both magnetic lineation and magnetic foliation oriented parallel to the regional fold axes in most of the cases the magnetic fabric acquired when the bedding was still horizontal is related to different degrees of lps shortening in some other cases the magnetic foliation is vertical and oblique to the bedding suggesting that in the ferdows thrust fold structure a cleavage system not visible at the outcrop scale has been developed as a consequence of later shortening related to the activity of the thrust system responsible of the recent earthquakes in the area results from this study establish a relationships between the recent tectonics of the area and the evolution of the magnetic fabric and demonstrate that a well defined magnetic fabric of pure tectonic origin can also develop in poorly deformed rocks lacking field evidence of pervasive internal deformation

the relationship between riverine u series disequilibria and erosion rates in a basaltic terrain u series isotopes have been measured in the dissolved phase suspended load and bedload of the main rivers draining basaltic catchments in iceland for the dissolved phase 234u 238u and 238u 230th range between 1 08 and 2 2 and 7 4 and 516 respectively for the suspended load and bedload 234u 238u and 238u 230th range from 0 97 to 1 09 and from 0 93 to 1 05 respectively chemical erosion rates calculated from dissolved major elements range between 13 and 333 t km 2 yr 1 physical erosion rates have also been estimated from existing data and range between 21 and 4864 t km2 yr with an average of 519 t km 2 yr 1 u series disequilibria indicate that weathering in iceland operates at close to steady state conditions a model of continuous weathering indicates a maximum weathering timescale of 10 kyr with an average rate of uranium release into water of 1 6 10 4 yr 1 which is significant when compared to granitic terrains located at similar latitudes and to tropical basaltic terrains all river waters display 234u 238u greater than secular equilibrium consistent with the effects of alpha recoil the same dissolved phase 234u 238u exhibit a negative trend with physical erosion rates explained by the dominant effect of close to congruent chemical weathering of hyaloclastites in the younger basaltic terrains therefore chemical erosion rate and mineral weathering susceptibility play a major role in determining 234u 238u disequilibria in basaltic river waters comparison of global data for river basins in which weathering was recently strongly limited indicates a negative correlation between silicate weathering rates estimated with major elements and the age of weathering estimated with u series disequilibria this strongly suggests a key role of time and soil thickness on the chemical erosion of silicates

the origin of tectonic lineation in extensional basins combined neutron texture and magnetic analyses on undeformed clays abstract in extensional sedimentary basins fine grained sediments that appear undeformed at the outcrop scale can carry a magnetic fabric consistent with the regional deformation pattern the origin of the magnetic lineation which is often found in extensional basins is not yet well understood in clays from extensional basins in southern italy the magnetic lineation is tectonically controlled and oriented perpendicular to the main normal faults a combined analysis of magnetic and mineral fabrics was made to gain insight into the processes that lead to a lineation in extensional settings low field high field and low temperature susceptibility measurements were used to distinguish the ferrimagnetic and paramagnetic contributions to the magnetic susceptibility and its anisotropy the magnetic anisotropy of the sediments is predominantly carried by paramagnetic phyllosilicates neutron texture analysis was used to evaluate the spatial distribution of chlorite basal planes results demonstrate that the orientation of the magnetic lineation is related to the spatial distribution of chlorite lying parallel to the common axis of differently oriented basal planes a quantitative correlation between the magnetic and rock fabric was made comparing the low and high field magnetic anisotropy ams hfa to the theoretical anisotropy calculated from the chlorite preferred orientation a good linear correlation is found between the degree of theoretical anisotropy and the ams and hfa results show that the integrated approach of magnetic and mineral fabric investigations represents a valid alternative tool for detecting grain scale and regional deformation patterns in weakly deformed extensional basins where macroscopic evidence of deformation is often not visible

comment on steady state 226ra 230th disequilibrium in mantle minerals implications for melt transport rates in island arcs by m d feineman and d j depaolo earth planet sci lett 215 2003 339 355

influence of cyclonic perturbations on surface winds around dumont d urville east antarctica using wavelet transform wavelet analyses of the surface winds at dumont d urville an east antarctic station have been performed for the months april june september and december 1993 representing autumn winter spring and summer respectively the 3 hourly 10 min averaged meteorological station data have been used the site is characterized by intense katabatic winds that arrive from the southeast during the year 1993 the maximum occurrence of wind was from 160 degrees direction sector the wavelet transform spectra of horizontal wind show prominent periodicities in the interval between 2 and 20 days the spectral energies in the diurnal scale are comparatively very low even in summer a comparison with wavelet spectra of pressure for synoptic scale 2 10 days confirms the influence of synoptic disturbances that are frequent in coastal regions of the antarctic continent the same analysis carried out on the wind components parallel and transverse to the main katabatic flow direction reveals that these periodicities are mainly due to the katabatic flow behavior although the katabatic wind flow is the dominant force characterizing the day to day weather conditions the influence of transient cyclones offshore cannot be ignored in situations of considerable synoptic activity the intensity and duration of katabatic wind flow are prolonged

upper cretaceous to eocene pelagic limestones of the scaglia rossa are not miocene turbidites the scaglia rossa is a pink limestone composed of 1 20 forams and 5 clay in a coccolith matrix that outcrops extensively in the umbria marches apennines of northern peninsular italy it has universally been considered to be a pelagic sedimentary rock deposited during the late cretaceous to middle eocene the scaglia rossa has been studied intensively as a record of micropalaeontological1 7 palaeomagnetic8 30 sedimentological31 34 and geochemical33 35 39 events recently wezel40 reported that the limestones are not of pelagic origin but are entirely turbiditic and were deposited in the miocene he also criticized magnetic polarity stratigraphy investigations done on the scaglia rossa concluding that the polarity is a function of sedimentation rate not of geomagnetic field polarity his evidence has cast doubt on previous stratigraphie information from the scaglia rossa41 and compels us to respond although the observation by coccioni42 and wezel40 of the presence of fossils of anomalously young ages in clay seams between the limestone beds is of interest we show here that the proposed redating of the scaglia is wrong for several palaeontological sedimentological and palaeomagnetic reasons

oxygen isotope constraints on the petrogenesis of aleutian arc magmas the first measurement of 18 o 16 o ratios of plagioclase clinopyroxene orthopyroxene and titanomagnetite phenocrysts from modern aleutian island arc lavas provides new insight and independent constraints on magma sources and intracrustal processes basalts are heterogeneous on the scale of the entire arc 18 oplag 5 5 6 7 and individual volcanic centers ranges of 0 3 to 0 8 combined with sr isotope and trace element data 18 oplag values suggest a variable magma source characterized by differences in the mantle wedge or the subducted sediment component along the volcanic front seven tholeiitic basalt to rhyodacite 50 71 sio 2 lavas from the seguam volcanic center have nearly identical 18 oplag values of 6 0 0 2 reflecting extensive closed system plagioclase dominated crystal fractionation oxygen isotope thermometry and pyroxene and oxide equilibria indicate that differentiation occurred between 1150 100 c basalt and 950 100 c rhyodacite in contrast 18 oplag values of 12 calc alkalic basaltic andesites and andesites 53 62 sio 2 from the smaller kanaga volcanic center span a broader range of 5 9 6 6 and consist of mostly higher values isotopic disequilibrium in the kanaga system is manifest in two ways two types of basaltic inclusions with contrasting 18 o values 6 0 and 6 5 occur in one andesite 18 oplag 6 6 and in two other andesites plagioclase titanomagnetite and clinopyroxene titanomagnetite oxygen isotope temperatures are inconsistent one andesite however yields concordant oxygen isotope temperatures of 850 100 c and a two pyroxene temperature of 920 50 c assimilation of isotopically heavy crustal wall rock and mixing off basaltic and andesitic magmas explains the variable and high 18 oplag values and low eruptive temperature of the kanaga andesites

case studies of plagioclase growth and residence times in island arc lavas from tonga and the lesser antilles and a model to reconcile discordant age information abstract the time scales of crystal growth and residence in magmas have received much attention because such information can constrain the physical processes involved however crystals and liquids can have different ages leading to apparently contradictory information from different dating methods two case studies are presented from island arc lavas that highlight such dichotomies plagioclase crystal size distributions and u th ra isotope data are presented for lavas from late and metis shoal in tonga and from soufriere st vincent in the lesser antilles in all three lavas mineral data yield apparent u th ages of 30 50 kyr yet 226 ra excesses are preserved in both the minerals and the whole rocks the crystal size distributions suggest crystal growth over 100s of years but also frequently indicate the presence of mixed crystal populations furthermore a cumulate from soufriere yields the same u th age as its host lava on 226 ra ba versus time evolution diagrams curves for melts in equilibrium with plagioclase provide apparent ages of 1700 3300 yr for tonga but for soufriere they are displaced below that of the whole rock our interpretation is that the phenocrysts from all three lavas are zoned in both composition and age the apparent u th ages reflect mixtures of old cores and young rims and indicate a volume integrated age of 10s of kyr for the time for initial crystal growth crystal liquid segregation the time of storage as a cumulate plus the time spent during entrainment and growth in the new magma prior to eruption the 226 ra excesses were imparted to overgrowths on the old cumulate cores during entrainment by subsequent injections of new magma because crystal growth segregation and entrainment can all appear to occur in 100s to a few thousand years at most the bulk of the u th time scale is inferred to record storage in cumulates which are then incorporated into new magma batches plagioclase growth rates of 3 9 10 13 cm s 1 would reconcile the combined ra th and crystal size distribution data magma differentiation time scales and eruptive periodicity are one to three orders of magnitude shorter than the inferred cumulate storage times indicating complex phenocryst liquid behaviour beneath island arc volcanoes

potassium argon geochronology of a basalt andesite dacite arc system the mount adams volcanic field cascade range of southern washington high precision k ar dating and detailed mapping have established an eruptive chronology for a cascade stratovolcano and its surrounding array of coeval basaltic centers mount adams is a 200 km 3 andesitic cone that stands at the center of a coeval 1250 km 2 quaternary volcanic field that contains 60 discrete vents k ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping within analytical error there are no discrepancies between k ar ages and stratigraphic sequence major activity began in the area ca 940 ka and inception of the central stratovolcano took place at ca 520 ka a plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was 3 k y and 80 of the products were basaltic andesites are volumetrically dominant and were emplaced in three main cone building episodes centered at 500 450 and 30 ka at eruptive rates of 1 6 5 km 3 k y at a lower rate of 0 05 0 1 km 3 k y the magmatic system remained almost continuously active between the main pulses although breaks in activity as long as 30 k y are permitted by the k ar data andesitic dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k y and their products are interstratified the last main episode of cone construction occurred ca 40 10 ka the oldest andesite identifiably derived from within the present day edifice having an age of 33 14 ka andesites forming the south summit rim and the true summit have ages of 13 8 ka and 15 8 ka respectively the time volume composition data bear upon several fundamental questions concerning the long term behavior of arc volcanoes stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity large stratocone systems may remain active for half a million years subdivision of complex stratovolcanoes into eruptive or constructional stages without detailed mapping geochronology and compositional data should be treated skeptically discussion of volumetric eruptive rates can be misleading without an adequate time scale stratovolcanoes need never develop large upper crustal magma chambers basalt erupts peripherally but its ascent is suppressed beneath stratovolcanoes by deep crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting

rapid remobilization of magmatic crystals kept in cold storage we lack thermal histories for magma reservoirs but here the magma under mount hood oregon usa is shown to have been too cold to mobilize for most of the time it has been stored which implies that magma mobilizes at which point it can be imaged geophysically very quickly prior to eruption this paper presents a novel approach to constraining the thermal history of the subsurface magma bodies that feed eruptions by combining temporal information from a variety of sources kari cooper and adam kent combine timescales derived from uranium series disequilibria textural information and trace element zoning in crystals they show that at mount hood oregon only a small fraction of the total storage duration has been spent at temperatures above the critical crystallinity the point at which magma is easily mobilized they conclude that largely liquid bodies that could be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption the processes involved in the formation and storage of magma within the earth s upper crust are of fundamental importance to volcanology many volcanic eruptions including some of the largest result from the eruption of components stored for tens to hundreds of thousands of years before eruption1 2 3 although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes they remain relatively poorly known4 5 eruptions of crystal rich magma are often suggested to require the mobilization of magma stored at near solidus conditions6 7 8 however accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures7 8 9 what has been lacking in this debate is clear observational evidence linking the thermal and therefore physical conditions within a magma reservoir to timescales of storage that is thermal histories here we present a method of constraining such thermal histories by combining timescales derived from uranium series disequilibria crystal sizes and trace element zoning in crystals at mount hood oregon usa only a small fraction of the total magma storage duration at most 12 per cent and probably much less than 1 per cent has been spent at temperatures above the critical crystallinity 40 50 per cent at which magma is easily mobilized partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near solidus temperatures is common magma storage at low temperatures indicates that although thermobarometry calculations based on mineral compositions may record the conditions of crystallization they are unlikely to reflect the conditions of most of the time that the magma is stored our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption

eruptive history geochronology and magmatic evolution of the puyehue cordon caulle volcanic complex chile forty three 40 ar 39 ar age determinations of lava flows domes ignimbrites and dikes plus 14 c dates from seven distal tephra layers combined with stratigraphy geochemistry and sr and th isotope data establish an eruptive chronology for the puyehue cordon caulle volcanic complex at 40 5 s in the andean southern volcanic zone svz the complex preserves 131 km 3 of lava and tephra that erupted from numerous vents widely separated in time and space approximately 80 of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka the modern puyehue stratovolcano was built on the southerly shield during the past 69 k y following a hiatus of 25 k y puyehue has erupted 15 km 3 of basaltic to rhyolitic magma that spans the entire compositional range found in the southern svz and evolved via at least six phases including 1 basaltic andesitic to dacitic lavas between 69 and 32 ka 2 a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of mgo rich basaltic andesite at 34 ka 3 dacitic to rhyolitic flows and domes from 19 to 12 ka 4 basaltic to basaltic andesitic flows between 15 and 12 ka 5 subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka followed by 6 a powerful series of phreatomagmatic and sub plinian eruptions at ca 1 1 ka that obliterated the preceding rhyolite domes and formed the present 2 5 km diameter 280 m deep summit crater along the cordon caulle fissure zone 5 km 3 of rhyodacitic to rhyolitic lavas domes and cones have formed during the past 16 5 k y including explosive and effusive eruptions in 1921 1922 and 1960 eruptive rates were nonuniform over time with background growth at 0 04 km 3 k y or less punctuated by spurts at up to 0 90 km 3 k y the time averaged rate 0 42 km 3 k y is nearly double that at the tatara san pedro complex 500 km to the north during the past 300 k y these findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable the last three stratocone building events on puyehue began during periods of deglaciation suggesting a relationship between unloading of ice and ease of magma ascent puyehue basalt exhibits subtle changes in 238 u 230 th 87 sr 86 sr and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge or the extent to which basalt was modified by assimilation of heterogeneous crustal melts the complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt despite the high flux of basalt during the past 300 k y no large silicic magma reservoir formed in the upper crust instead 238 u 230 th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust promoted perhaps by conduits that reflect strike slip faulting beneath the complex

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

magnetic stratigraphy applied to synsedimentary slumps turbidites and basin analysis the scaglia limestone at furlo italy the previously studied italian scaglia sections at gubbio and moria consist entirely of pelagic limestone and marl by contrast the furlo section is characterized by synsedimentary slump folds and slide masses in the lower portion cenomanian coniacian and by intercalation of white calcarenitic turbidites in the upper part campanian paleocene demagnetization to 20 mt and 350 c or higher reveals a single stable direction after removal of an initial low coercivity component despite the stratigraphic complications the pattern of polarity zones clearly matches gubbio and the sea floor anomalies the abrupt faunal break at the end of the cretaceous was located and foram zonation shows that the very base of the paleocene is missing sedimentation rate determinations based on the magnetic polarity zonation have clarified the origin of stratigraphic complications in the furlo paleocene most samples were taken from pelagic beds but ten samples from calcarenitic turbidites yielded directions indistinguishable from those of adjacent pelagic beds detailed sampling of a turbidite 33 cm thick confirmed that this lithology is an accurate recorder of the geomagnetic field synsedimentary slumps range from coherent slide blocks with undeformed bedding to smoothly curving folds indicating flow of very soft sediment coherent slide blocks show declinations rotated by tens of degrees away from the typical regional direction but their inclinations are unaffected soft sediment slump folds yield typical regional directions indicating resetting of the remanent magnetization after deformation using stratigraphic structural and paleomagnetic data it is possible to identify in the lower part of the section a basal nonslumped unit overlain by seven slump units with various characteristics inter layered with four units of pelagic sediments which were deposited on older slump units but which themselves were not affected by synsedimentary slumping this paper emphasizes the value of paleomagnetic stratigraphy as a tool in detailed studies of sedimentology stratigraphy and basin analysis

a tectonic origin of magnetic fabric in the shemshak group from alborz mts northern iran abstract the magnetic lineation observed in undeformed sedimentary units has been interpreted either as an indication of paleoflow direction or as a result of tectonic overprint which progressively modifies the original sedimentary fabric related to compactional processes distinguishing between the two processes is not always easy in fact most studies of the anistropy of magnetic susceptibility ams of undeformed sequences have been carried out in fine grained sediments from foredeep sequences which are characterized by sedimentary flow directions which are almost parallel to the main deformation structures like thrust faults and folds in the alborz mts the upper triassic lower jurassic shemshak group was deposited in a foreland to molassic basin of the eo cimmerian orogen and now outcrops in several folds which are oriented parallel to the curved chain paleoflow directions are generally oblique to the main tectonic structures being directed ssw to sse and showing negligible changes in their orientation along the alborz mountains we have therefore the opportunity to distinguish between tectonic or sedimentary related origins of the magnetic lineation the ams results show that magnetic lineations of the shemshak group are oriented almost parallel to the main fold axes and thrust structures which follow the alborz mts curved trend suggesting that magnetic lineation is of tectonic origin in fine to medium grained mostly massive sandstones and confirming that ams is a valuable tool to study deformation processes in sedimentary rocks

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

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

a study of rare earth element ree sio 2 variations in felsic liquids generated by basalt fractionation and amphibolite melting a potential test for discriminating between the two different processes the origin of felsic magmas 63 sio2 in intra oceanic arc settings is still a matter of debate two very different processes are currently invoked to explain their origin these include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite because both fractionation and melting can lead to similar major element trace element and isotopic characteristics in felsic magmas such lines of evidence have been generally unsuccessful in discriminating between the two processes a commonly under appreciated aspect of rare earth element ree solid liquid partitioning behavior is that d ree for most common igneous minerals especially hornblende increase significantly with increasing liquid sio2 contents for some minerals e g hornblende and augite ree partitioning can change from incomptatible d 1 at high liquid sio2 when this behavior is incorporated into carefully constrained mass balance models for mafic basaltic amphibolite melting intermediate andesitic amphibolite melting lower or mid to upper crustal hornblende present basalt fractionation and mid to upper crustal hornblende absent basalt fractionation the following general predictions emerge for felsic magmas e g 63 to 76 sio2 partial melting of either mafic or intermediate amphibolite should regardless of the type of melting equilibrium fractional accumulated fractional yield ree abundances that remain essentially constant and then decrease or steadily decrease with increasing liquid sio2 content at high liquid sio2 contents lree abundances should be slightly enriched to slightly depleted i e c l c o 2 to 0 2 while hree abundances should be slightly depleted c l c o 1 to 0 2 lower crustal hornblende bearing basalt fractionation should yield roughly constant ree abundances with increasing liquid sio2 and exhibit only slight enrichment c l c o 1 2 mid to upper crustal hornblende bearing basalt fractionation should yield steadily increasing lree abundances but constant and then decreasing hree abundances at high liquid sio2 contents lree abundances may range from non enriched to highly enriched c l c o 1 to 5 while hree abundances are generally non enriched to only slightly enriched c l c o 1 to 2 hornblende absent basalt fractionation should yield steadily increasing ree abundances with increasing liquid sio2 contents at high sio2 contents both lree and hree are highly enriched c l c o 3 to 4 it is proposed that these model predictions constitute a viable test for determining a fractionation or amphibolite melting origin for felsic magmas in intra oceanic arc environments where continental crust is absent

plagioclase residence times at two island arc volcanoes kameni islands santorini and soufriere st vincent determined by sr diffusion systematics the diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase melt partition coefficient trace element diffusion will subsequently act to modify this initial profile and given enough time produce a profile that is in equilibrium with the anorthite variations within the crystal we argue that the trace element partition coefficient da b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal liquid partition coefficients and that the equilibrium profile of the crystal can be calculated the time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase strontium plagioclase melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions a one dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir the model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the kameni island dacites santorini and from the 1979 soufriere andesite st vincent three out of eight plagioclase phenocrysts have sr concentration profiles that are not in diffusive equilibrium for these the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase maximum crystal residence times range from 100 to 450 years this is in good agreement with estimates from crystal size distribution and from ra th disequilibrium studies for the kameni islands for soufriere however such short residence times are incompatible with u th mineral errorchron data that suggest residence times of 40 ka in a thermally buffered magma reservoir to reconcile these apparently different ages we invoke a more complicated magmatic history for soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption

timescales of crustal magma reservoir processes insights from u series crystal ages the dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas and crystals in volcanic rocks provide a temporally constrained archive of these changing environments in this review i compile 238 u 230 th ages of accessory phases and 238 u 230 th 226 ra ages of bulk mineral separates of major phases these data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre age populations for accessory phases show protracted pre eruptive crystal residence times but few crystals predate magmatic activity at a given centre these data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system in contrast less than half of the bulk separate 238 u 230 th 226 ra ages for major phases are more than 10 kyr older than the eruption many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption overall the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs especially when combined with other crystal scale geochemical data

contrasting timescales of crystallization and magma storage beneath the aleutian island arc abstract geologic chronologic and u th isotope data from pleistocene recent basaltic to rhyolitic lavas and their phenocrysts constrain the long term evolution of magmatic processes at seguam island aleutian island arc and suggest a model in which erupted magmas were derived from a single deep seated reservoir for 130 kyrs of its eruptive history the monotonic evolution in 230 th 232 th 0 ratios is consistent with radiogenic ingrowth of 230 th in a long lived magma reservoir between 142 and 9 ka internal u th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40 ar 39 ar dating which implies a short period i e 10 3 yrs or less of crystal residence in the magma prior to eruption these results can be reconciled if small batches of magma are repeatedly extracted from a deep thermally buffered basaltic reservoir followed by decompression driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering 12 km 2 the ignimbrite as well as subsequent rhyolitic and basaltic lava flows signal a decrease in 230 th 232 th 0 and 87 sr 86 sr ratios the abrupt change in magma composition at 9 ka most likely reflects disruption of the long lived reservoir by rapid ascent of basaltic magma with low 230 th 232 th and 87 sr 86 sr ratios and excess 226 ra while the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored the shift in isotopic composition and evidence for a protracted 100 kyr period of magma storage would have been missed had the focus been on only the most recent historical eruptions our results suggest that not all arc basalt produced in the mantle wedge separates and ascends rapidly magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano and u th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs provided a sufficiently long period of volcanic activity is examined

anisotropy of magnetic susceptibility in the scaglia rossa pelagic limestone abstract during folding of the scaglia rossa limestone in umbria italy deformation was mainly accommodated by pressure solution cleavage fossils between the cleavage planes appear visibly undeformed yet the limestone possesses a weak magnetic fabric the maximum and intermediate principal axes of the magnetic anisotropy ellipsoid define a distinct magnetic foliation plane within which a weak concentration of the maximum axes forms a magnetic lineation neither of these features is of sedimentary origin results from a slumped outcrop where bedding and a cleavage induced by overburden compaction have different attitudes show that the magnetic foliation is caused by the compaction comparisons with field derived structural data suggest that the magnetic lineation was produced tectonically during deformation of the apennine fold belt

magnetic fabric analysis as a tool to constrain mechanisms of deep water mudstone deposition in the marnoso arenacea formation miocene italy abstract mudstone dominated marine successions are common in the geological record yet a full understanding of their depositional processes is often hampered by a lack of generally accepted diagnostic criteria to distinguish between hemipelagic settling and deposition from a flowing medium the marnoso arenacea formation a turbidite unit of miocene age cropping out in the northern apennines of italy offers the possibility to address some of these uncertainties a relatively small 10 but distinctive portion of the marnoso arenacea formation is composed of white marlstone beds wm beds that have frequently been interpreted as due to hemipelagic settling of fine grained particles hemipelagites the analysis of the anisotropy of magnetic susceptibility ams revealed the presence in the wm beds of maximum susceptibility axes clustered within the depositional plane along the average paleoflow direction inferred from flute casts at the bases of the nearest turbidite beds whereas the minimum susceptibility axes are oriented perpendicular to the bedding plane this fabric is interpreted as largely sedimentary in origin albeit a contribution from tectonic shortening cannot be excluded and due to the alignment within the bedding plane of paramagnetic grains e g muscovite and possibly also ferromagnetic grains magnetite under low velocity currents the trend of the maximum susceptibility axes and hence of the paleoflow direction is approximately oriented nnw sse after correction for apennines thrust sheet rotation since the miocene these results suggest that the wm beds cannot be entirely due to hemipelagic settling as often stated in the literature a discussion of alternative depositional mechanisms leads us to conclude that the wm beds may have been deposited under the influence of contour currents and should therefore be referred to as muddy contourites

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

the effect of assimilation fractional crystallization and ageing on u series disequilibria in subduction zone lavas abstract although most arc lavas have experienced significant magma differentiation the effect of the differentiation process on u series disequilibria is still poorly understood here we present a numerical model for simulating the effect of time dependent magma differentiation processes on u series disequilibria in lavas from convergent margins our model shows that in a closed system with fractional crystallization the ageing effect can decrease u series disequilibria via radioactive decay while in an open system both ageing and bulk assimilation of old crustal material serve to reduce the primary u series disequilibria in contrast with recharge of refresh magma significant 226ra excess in erupted lavas can be maintained even if the average residence time is longer than 8000 years the positive correlations of 226ra 230th between sr th or ba th in young lavas from convergent margins have been widely used as evidence of fluid addition generating the observed 226ra excess in subduction zones we assess to what extent the positive correlations of 226ra 230th with sr th and ba th observed in the tonga arc could reflect afc process results of our model show that these positive correlations can be produced during time dependent magma differentiation at shallow crustal levels specifically fractional crystallization of plagioclase and amphibole coupled with contemporaneous decay of 226ra can produce positive correlations between 226ra 230th and sr th or ba th to a lesser extent therefore the correlations of 226ra 230th with sr th and ba th cannot be used to unambiguously support the fluid addition model and the strength of previous conclusions regarding recent fluid addition and ultra fast ascent rates of arc magmas is significantly lessened

transient volcano deformation sources imaged with interferometric synthetic aperture radar application to seguam island alaska 1 thirty interferometric synthetic aperture radar insar images spanning various intervals during 1992 2000 document coeruptive and posteruptive deformation of the 1992 1993 eruption on seguam island alaska a procedure that combines standard damped least squares inverse methods and collective surfaces identifies three dominant amorphous clusters of deformation point sources predictions generated from these three point source clusters account for both the spatial and temporal complexity of the deformation patterns of the insar data regularized time series of source strength attribute a distinctive transient behavior to each of the three source clusters a model that combines magma influx thermoelastic relaxation poroelastic effects and petrologic data accounts for the transient interrelated behavior of the source clusters and the observed deformation basaltic magma pulses which flow into a storage chamber residing in the lower crust drive this deformational system a portion of a magma pulse is injected into the upper crust and remains in storage during both coeruption and posteruption intervals this injected magma degasses and the volatile products accumulate in a shallow poroelastic storage chamber during the eruption another portion of the magma pulse is transported directly to the surface via a conduit roughly centered beneath pyre peak on the west side of the island a small amount of this magma remains in storage during the eruption and posteruption thermoelastic contraction ensues this model made possible by the excellent spatial and temporal coverage of the insar data reveals a relatively simple system of interrelated predictable processes driven by magma dynamics

the volcanoes of an oceanic arc from origin to destruction a case from the northern luzon arc abstract volcanoes were created grew uplifted became dormant or extinct and were accreted as part of continents during continuous arc continent collision volcanic rocks in eastern taiwan s coastal range cr are part of the northern luzon arc an oceanic island arc produced by the subduction of the south china sea plate beneath the philippine sea plate igneous rocks are characterized by intrusive bodies lava and pyroclastic flows and volcaniclastic rocks with minor tephra deposits based on volcanic facies associations sr nd isotopic geochemistry and the geography of the region four volcanoes were identified in the cr yuemei chimei chengkuangao and tuluanshan near vent facies associations show different degrees of erosion in the volcanic edifices for chimei chengkuangao and tuluanshan yuemei lacks near vent rocks implying that yuemei s main volcanic body may have been subducted at the ryukyu trench with the northward motion of the philippine sea plate these data suggest a hypothesis for the evolution of volcanism and geomorphology during arc growth and ensuing arc continent collision in the northern luzon arc which suggests that these volcanoes were formed from the seafloor emerging as islands during arc volcanism they then became dormant or extinct during collision and finally were uplifted and accreted by additional collision the oldest volcano yuemei may have already been subducted into the ryukyu trench

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

timing of diagenetic haematite growth in red pelagic limestones from gubbio italy abstract magnetite haematite and to a minor extent maghaemite are recognised in the cretaceous and paleocene red pelagic limestones at gubbio the magnetite is detrital or biological whereas the haematite grew during diagenesis from a goethitic precursor thermal and af demagnetization of samples collected from close to reversal boundaries indicate that the various magnetization components do not record the polarity reversal at exactly the same stratigraphic level in the few tens of centimetersbelow a recorded geomagnetic reversal defined by the magnetite magnetization some of the haematite grains are magnetized in the post reversal field the blocking temperature spectra of this haematite fraction with post reversal magnetization are found to shift toward higher temperatures as the reversal boundary is approached the blocking temperature spectra reflect the grain size spectra of the haematite which we interpret as arising by the continual nucleation of grains down to a certain burial depth where the conditions are no longer conducive to further haematite growth the depth below reversal boundaries to which haematite with post reversal magnetization can occur is estimated to be about 60 cm after compaction and is equivalent to a time of about 105 years for these particular sediments a detailed study of the magnetization components at reversal boundaries indicates that the first diagenetic growth of haematite through the single domain critical volume occurs prior to the mechanical fixation of the detrital or biological magnetite subsequently the diagenetic haematite grains do not rotate in response to the ambient geomagnetic field polarity as easily as the magnetite because of their occurrence as pigmentary coatings on larger non magnetic grains

neogene quaternary evolution of the central apennine orogenic system italy a structural and palaeomagnetic approach in the molise region abstract we report on new palaeomagnetic and magnetic fabric analyses of mainly upper miocene sedimentary sequences from the external central apennine fold and thrust belt molise area where the principal compressive structures are clearly non coaxial the sampling was carried out on the e w oriented matese frosolone thrust sheet that for its geographical position and structural setting superposition of thrusting strike slip and extensional tectonics since late miocene to present day represents a key structure for the comprehension of the neogene quaternary evolution of the entire molise area palaeomagnetic results suggest that the matese frosolone thrust sheet counterclockwise rotated at least 35 after messinian times these data confirm that the present day trend variability observed in the main compressional structures in the apennine chain can be related to rotations about vertical axes rather than to changes in the stress field orientation at least since late miocene times magnetic fabric analyses indicate that the studied sediments were subjected to very mild deformation suggesting that the surface emergence of the thrust front of the matese frosolone unit is located farther north far from the studied area well defined magnetic lineations of tectonic origin were only observed in sites close to localised belts of strike slip deformation

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

40ar 39ar k ar and 230th 238u dating of the laschamp excursion a radioisotopic tie point for ice core and climate chronologies abstract a brief period of enhanced 10 be flux that straddles the interstadial warm period known as dansgaard oeschger event 10 in greenland and its counterpart in antarctica the antarctic isotope maximum 10 is but one consequence of the weakening of earth s magnetic field associated with the laschamp excursion this 10 be peak measured in the grip ice core is dated at 41 250 y b2k before year 2000 ad in the most recent gicc05 age model obtained from the northgrip core via multi parameter counting of annual layers uncertainty in the age of the 10 be peak is however no better than 1630 y at the 95 confidence level reflecting accumulated error in identifying annual layers the age of the laschamp excursion guillou h singer b s laj c kissel c scaillet s jicha b 2004 on the age of the laschamp geomagnetic excursion earth planet sci lett 227 331 343 is revised on the basis of new 40 ar 39 ar unspiked k ar and 238 u 230 th data from three lava flows in the massif central france together with the 40 ar 39 ar age of a transitionally magnetized lava flow at auckland new zealand combined these data yield an age of 40 700 950 y b2k where the uncertainty includes both analytical and systematic 40 k and 230 th decay constant errors taking the radioisotopic age as a calibration tie point suggests that the layer counting chronologies for the northgrip and gisp2 ice cores are more accurate and precise than previously thought at depths corresponding to the laschamp excursion

synsedimentary slides and bedding formation in apennine pelagic limestones abstract jurassic to lower tertiary pelagic carbonates in the umbria marches apennines of northern peninsular italy are well bedded and synsedimentary submarine slide deposits are common bedding planes were formed by pressure solution during compaction where original rhythmic alternations in clay content were present bedding formed along the clay rich horizons where the original sediments were uniform stylolitic pseudobedding approximately marks paleohorizontal even where true bedding departs from paleohorizontal because of the presence of soft sediment folds deformational facies observed in synsedimentary slides are compared to a conceptual model of a bedding plane glide with a longitudinal sequence of four deformational facies 1 the slide scar where section is missing may be identified if biostratigraphic or magnetostratigraphic zones are missing 2 the stretched rear of the slide is thinned and shows extensional shear joints and sedimentary boudinage 3 the transported but undeformed main body of the slide may be identified paleomagnetically if it has been rotated and the basal shear surface may show deformational structures 4 the shortened front of the slide commonly shows on a small scale the ramp thrusts and folds typical of foreland fold and thrust belts the state of induration of the sediments at the time of movemen is shown by their theological behavior in areas of major subsea relief slides may have moved far enough for the distal parts to lose their coherence and all the transitional stages from a slide to a pebbly mudstone can be identified

bioturbation minimal effects on the magnetic fabric of some natural and experimental sediments magnetic fabric measurements have been performed on three suites of sediment samples to contrast biologically disturbed and undisturbed sediments the first of these analyses includes samples from laminated relatively undisturbed and bioturbated totally disturbed horizons found in an athabasca oil sand core the second includes samples from biologically undisturbed sediments from mammoth cave in kentucky the third includes evaluation of the evolution of a magnetite horizon which was established by the author in december of 1982 in a biologically active tidal flat on sapelo island georgia changes in this magnetite horizon were monitored by periodic subsampling results from these suites of samples indicate that the measured magnetic fabric in bioturbated sediments may exhibit primary sedimentary characteristics similar to undisturbed sediments such primary indicators include the anisotropy of magnetic susceptibility ams parametersv used by graham always greater than 45 q used by rees and hamilton always less than 0 69 and a subhorizontal magnetic foliation plane i infer that the presence of apparently primary fabric distributions in these bioturbated sediments indicates that physical not biological factors are responsible for the observed fabric these factors include dewatering and simple low magnitude compaction the data indicate that it cannot be assumed a priori that poor fabric distributions are the result of bioturbation on the contrary for the sapelo island experiments the magnetic fabric actually improved with bioturbation and the athabasca bioturbated magnetic fabric is very similar to that from the athabasca laminated sediment data from the sapelo island georgia tidal flat experiment are interpreted to indicate that long axis ams alignments develop within the sediment such alignments are the result of water movement through sediment which exhibits increased porosity due to high rates of burrowing it is also observed that rm precision improves during the experiment in spite of bioturbation

magma cumulate mixing identified by u th disequilibrium dating magma mixing is believed to be important in magmagenesis1 particularly in volcanic arcs2 where its influence is revealed by heterogeneous phenocryst populations3 and quenched inclusions4 nevertheless isotope studies particularly of uranium series dis equilibria have concentrated on the analysis of bulk rock samples5 9 comprehensive u th mineral separate data from arc volcanics are scarce10 14 here we present the first u th isotopic evidence for mixing between a crystal mush and a magma in an andesite from the island of santorini in the aegean arc the lava contains crystal populations from two sources of distinct thorium isotope composition one from a basic cumulate the other phenocrysts from a dacite magma the crystal populations have statistically indistinguishable crystallization ages of 79 14 12kyr and 93 29 22kyr and are thought to have physically intermingled just before eruption the crystals did not re equilibrate and hence retain the isotopic compositions and ages of the parental materials

new age for the skalamaelifell excursion and identification of a global geomagnetic event in the late brunhes chron article ar 39 ar geochronology paleomagnetism sixteen lava flows sampled from isolated hills including skalamaelifell hill on the reykjanes peninsula in southwest iceland record excursional geomagnetic field directions that correspond to virtual geomagnetic poles vgps grouped at 12 s 250 e in the eastern pacific ocean the vgps are similar to those obtained in earlier studies of these lavas for which the name skalamaelifell excursion was previously assigned based on unspiked k ar dating of nine samples from six hills it has been accepted for more than two decades that the transitionally magnetized lavas reflect the 41 ka laschamp excursion however nine new 40 ar 39 ar incremental heating experiments on four of the transitionally magnetized lavas give an age of 91 13 ka moreover 238 u 230 th isochrons determined for two of these lava flows yield a weighted mean age of 96 10 ka 2 the weighted meanof the

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods

constraints on deformation mechanisms during folding provided by rock physical properties a case study at sheep mountain anticline wyoming usa summary the sheep mountain anticline wyoming usa is a well exposed asymmetric basementcored anticline that formed during the laramide orogeny in the early tertiary in order to unravel the history of strain during folding we carried out combined anisotropy of magnetic susceptibility ams anisotropy of p wave velocity apwv and fry strain analyses the results are compared to previously published stress strain data from calcite twins at the microscopic scale and from fracture sets at the mesoscopic scale and are used to discuss the kinematics and mechanics of forced folding the results obtained in sandstone and carbonate lithologies demonstrate a good agreement between 1 the principal axes of the ams and apwv tensors 2 stress strain tensors derived from calcite twins 3 fry strain axes and mesoscopic fracture sets furthermore these tensors are coaxial with the main structural trends of the anticline the differences between ams and apwv fabrics on one hand and the differential stress values of the forelimb and the backlimb on the other hand emphasize how the macroscopic asymmetry of sheep mountain anticline affects the strain pattern at the microscopic scale the data set presented in this paper offers a consistent mechanical scenario for the development of sheep mountain anticline