the heating duration and provenance age of rocks in the salton sea geothermal field southern california abstract 40ar 39ar step heating of single k feldspar crystals 500 g in a furnace with a low argon blank and accurate temperature control provide constraints on the thermal history of the salton sea geothermal field southern california estimates of 40ar loss from detrital k feldspars coupled with kinetic information gleaned from laboratory degassing of 39ar allow possible thermal histories to be evaluated samples from core and cuttings of sandstones from the state 2 14 landers 1 dearborn farms 1 magmamax 2 elmore 3 and sinclair 3 wells have been analyzed for this purpose no recent 40ar loss is recognized in any of the 17 crystals measured even for samples currently at temperatures greater than 275 c this observation suggests that the present temperature distributions in these wells have not been maintained for greater than 1000 years however aggregates 100 mg of two k feldspar samples do reveal recent 40ar loss from the state 2 14 well suggesting a time scale of 1000 to 5000 years for near peak thermal conditions these estimates of heating duration are broadly consistent with earlier results from thermal modelling but do not support interpretations that paleotemperatures were once hotter than present provenance ages span the tertiary through middle proterozoic but cluster at ages of 25 70 and 1250 ma these age peaks are very common k feldspar k ar ages in detachment terranes of southeastern california and southwestern arizona both drained by the colorado river this coincidence supports earlier suggestions that the sands of the salton trough were derived mainly from the colorado river unrealistically low apparent activation energies

petrology and isotope geochemistry of san vincenzo rhyolites tuscany italy two groups of rhyolites have been recognized at san vincenzo tuscany italy group a rhyolites are characterized by plagioclase quartz biotite sanidine and cordierite mineral assemblages they show constant mgo and variable cao and na2o contents initial87sr 86sr ratios in group a samples range between 0 71950 and 0 72535 whereas the nd isotopic compositions are relatively constant 0 51215 0 51222 group b rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals and show textural mineralogical and chemical evidence of interaction with more mafic magmas the sr and nd isotopic ratios range between 0 71283 0 71542 and 0 51224 0 51227 respectively magmatic inclusions of variable size 1 mm to 10 cm were found in groups b rhyolites these inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite they are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts these magmatic inclusions have relatively high sr contents 996 1529 ppm and sr and nd isotope ratios of 0 70807 0 70830 and 0 51245 0 51252 respectively 87sr 87sr data on minerals separated from both group a and b rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group a rhyolites and due to interaction of rhyolites with a mantle de rived magma in group b rhyolites isotopic data on whole rocks and minerals allow us to interpret the group a rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source conversely group b rhyolites underwent interactions with a mantle derived magma the crustal source as inferred from isotopic systematics would be characterized by87sr 86sr and143nd 144nd ratios close to 0 7194 and 0 51216 respectively the sub crustal magma would have sr isotopic composition close to 0 7077 and a143nd 144nd ratio greater than or equal to 0 51252 these isotopic features are different from those reported for the parental magmas postulated for vulsini and alban hills in the nearby roman magmatic province and are similar to those of the vesuvius and ischia magmas

evidence for li rich brines and early magmatic fluid rock interactionin the larderello geothermal system abstract the geochemical features of fluids accompanying the first stages of geothermal activity linked to magmatic intrusions have been documented for the larderello geothermal system italy deep drilling has provided samples which preserve evidence of this early geothermal activity four wells san pompeo 2 monteverdi 7 sasso 22 and serrazzano vc 11 penetrated the deeper parts of the larderello system located in a metamorphic basement underlying the tertiary nappe complex which constitutes the main aquifer at larderello the drill holes terminated close to the inferred roof of a granitic complex thought to be responsible for geothermal activity fluid inclusion data were obtained from recrystallized quartz lenses and quartz veins in samples displaying high temperature assemblages plagioclase actinolite biotite tourmaline clinopyroxene andradite wollastonite and also from magmatic quartz in a leucogranite dike the inclusions are mainly secondary in origin oriented in fluid inclusion planes fip related to hydrothermal circulation in the larderello system several generations of high temperature fluids were trapped and include 1 1 h2o co2 dominated vapors displaying variable but significant contents of ch4 and n2 2 2 aqueous vapors containing licl with variable salinity 3 3 aqueous licl brine often oversaturated with respect to halite at room temperature 4 4 complex brine always oversaturated at room temperature with respect to two halite and sylvite or more n 4 salts the presence of licl was confirmed by identification of the salt hydrate licl 5h2o at very low temperature using raman spectroscopy bulk salinities could be roughly estimated at around 30 wt eq licl for the licl brine geometric and chronologic relationships between fip reveal close relationships and mutual contamination between the h2o co2 vapors and licl brine indicating synchronism in their trapping these fluids were generated and trapped at pressures of 100 130 mpa nearly 23 mpa above the estimated present day lithostatic pressure this implies a denudation rate between 0 2 and 0 5 mrn a 1 since the onset of hydrothermal activity compatible with the setting of larderello in a young tortonian collision belt fluid inclusion trapping temperatures 425 650 c show a monotonous increase towards the inferred granite and are around 100 200 c higher than the highest present day temperatures the results are interpreted as recording the interaction between magmatic and contact metamorphic fluids in the early larderello system the h2o co2 vapors resulted from the reheating of the basement metamorphic series often c rich under relatively high temperatures during contact metamorphism lirich fluids expelled from an underlying li rich leucogranite migrated through the metamorphic series and the already cooled granite dikes and experienced local boiling the fluid inclusion data demonstrate the involvement of magmatic fluids during the initial development of this high energy geothermal field

direct measurement of ar diffusion profiles in a gem quality madagascar k feldspar using the ultra violet laser ablation microprobe uvlamp abstract controversy surrounding the mechanisms and controls on argon diffusion in k feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures over a range of temperatures measurements of argon diffusion profiles in a gem quality iron rich orthoclase heated in a cold seal apparatus have been undertaken in situ using an ultra violet laser ablation microprobe uvlamp technique the results agree very closely with the previously determined bulk values for benson mines orthoclase activation energy e 43 8 1 kcal mol 1 and vacuum furnace cycle heating studies of k feldspars e 46 6 kcal mol 1 however instead of defining a single activation energy e and diffusion coefficient do the data yield two sets of parameters a low temperature 550 720 c array with an e of 47 2 2 5 kcal mol 1 198 2 10 5 kj mol 1 and a do of 0 0374 0 1123 0 0281 cm2 s 1 and a high temperature 725 1019 c array with an e of 63 8 3 4 kcal mol 1 268 0 14 3 kj mol 1 and a do of 55 0 225 5 44 2 cm2 s 1 the new results closely reproduce two sets of apparent activation energies previously measured in cycle heating studies of madagascar k feldspar 40 3 and 57 3 kcal mol 1 previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion however the uv depth profile analyses indicate simple diffusion to the grain surface and importantly diffusion radii calculated by combining the uvlamp and cycle heating data are the same as the physical grain sizes used in the experiments around 1 mm vacuum furnace stepped heating experiments on slowly cooled k feldspars have been interpreted as showing diffusion radii of around 6 m and indicate complex populations of sub grains this study indicates that madagascar k feldspar and thus probably all gem quality k feldspars act as single diffusion domains and that short circuit or pipe diffusion was not an important loss mechanism an apparent diffusion compensation relationship in the stepped heating data for madagascar k feldspar implies that similar relationships seen in other k feldspars are a result of a range of diffusion mechanisms

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

fluid geochemical transect in the northern apennines central northern italy fluid genesis and migration and tectonic implications chemical and isotopic characteristics of natural gas and thermal water discharges from the western back arc tyrrhenian sea across the apennine thrust belt to the po valley and adriatic coast foredeep basins in the northern apennines central northern italy reveal a large scale fluid motion in the upper crust both vertically and horizontally on the basis of gas compositions two different domains of rising fluids have been distinguished 1 co2 rich he poor 3he 4he high domain in the western peri tyrrhenian extensional sector 2 ch4 rich he rich 3he 4he low domain in the eastern adriatic compressional sector such gases rising from various depths are crossed by a huge lateral n2 rich water flow in the peri tyrrhenian sector of ca so4 hco3 meteoric derived waters that move in a regional wide aquifer hosted in a quite thick mesozoic carbonate series morphologically the co2 vents consist of mud basins with high gas rate emission where the rising fluids move upwards through diatremes on the other hand ch4 emissions seep out from typical mud volcanoes with a reduced gas flow rate where the fluid motion is likely related to saline diapir extrusions and the methane is mostly carried to the surface by the associated mud the two rising mechanisms described locate southwest and northeast of the apennine watershed respectively from a seismic point of view the ch4 domain in the thrust belt and foredeep areas is characterized by a large number of earthquakes indirectly pointing to a different rheological behavior of the terrains with respect to the more internal peri tyrrhenian area owing to the quite high thermal gradient of the latter the boundary of the brittle ductile crust in the peri tyrrhenian sector can be located at a 10 km shallow depth although the presence of several post orogenic basins would suggest widespread extensional tectonics in all areas west of the apennine watershed those located in the easternmost part display gas vents with typical crustal 3he 4he ratios as this ratio is very sensitive to deep fluids rising from the mantle we hypothesize that such basins at the foot of the apennines are not due to tensive stress as suggested by their morphological shape they are piggy back thrust top basins developed and evolved in a still acting compressive tectonic regime

radiogenic isotopes in fluid inclusions radiogenic isotopes studied in fluid inclusions are still a limited field with great potential for expansion as analytical techniques improve the main limitation for sr ar and he isotope work is the very small number of radiogenic atoms produced in a typical fluid inclusion the requirements to analysts are correspondingly high examples show that isotopic tracing on fluid inclusion fluids can be a decisive tool in solving geological problems

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

factors controlling threshold friction velocity in semiarid and arid areas of the united states a physical model was developed to explain threshold friction velocities u t for particles of the size 60 120 m lying on a rough surface in loose soils for semiarid and arid parts of the united states the model corrected for the effect of momentum absorption by the nonerodible roughness for loose or disturbed soils the most important parameter that controls u t is the aerodynamic roughness height z0 for physical crusts damaged by wind the size of erodible crust pieces is important along with the roughness the presence of cyanobacteriallichen soil crusts roughens the surface and the biological fibrous growth aggregates soil particles only undisturbed sandy soils and disturbed soils of all types would be expected to be erodible in normal wind storms therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confirmed by our measurements

early fluid migration in a deep part of the larderello geothermal field a fluid inclusion study of the granite sill from well monteverdi 7 abstract metamorphic magmatic to hydrothermal fluids trapped in fluid inclusion planes fip were studied in a microfractured granite sill from a deep level 3483 3486 m b g l of the well monteverdi 7 larderello geothermal field this granite has recorded a complex succession of fluid migration through a dense set of healed microcracks nine generations of fluids trapped within specific microcrack networks are distinguished and include vapors and liquids of the c h o n s system aqueous liquids and brines with distinctive sets of cation chlorides licl nacl nacl cacl 2 and nacl detailed studies of the geometrical and chronological relationships between the fluid inclusion planes allow the establishment of a clear relative chronology with the c h o n s and licl fluids being earlier than the nacl cacl 2 and nacl aqueous liquids and brines in particular it can be shown that the c h o n s and licl fluids are strongly associated in space and time since the corresponding fluid inclusion planes display evidence of mutual contamination of inclusions within the planes significant changes in the physical chemical parameters occurred in the conditions of fluid trapping during the evolution of the larderello field all observed paleofluids have compositions differing from that encountered in the present day fluids and record a complex p t t x evolution from an early stage dominated by magmatic and metamorphic derived fluids towards a later stage dominated by more surficial waters at the early stage hydrothermal circulation occurred under lithostatic pressure at a depth of ca 4 5 km at temperatures as high as 450 500 c such conditions are very reminiscent of contact metamorphism and were probably created by the deep intrusive body detected by geophysical studies batini et al 1983 1985 the involved fluids are c h o n s vapors and liquids of metamorphic derivation and aqueous li bearing brines of probable magmatic derivation they are consistent with the hypothesis of the intrusive being a granite at the later stage physical conditions had evolved towards hydrostatic pressure at a depth close to the present 3 5 km one and temperatures of 400 350 c still higher than the present day ones early fluids were no longer present the observed fluids record the involvement of more surficial waters both meteoric and connate transient boiling events were probably occurring heralding the present day conditions vapor dominated geothermal system nevertheless no trapped fluids display similar composition and density with the present day geothermal fluids at larderello in old geothermal fields such as larderello it must be emphasized that the use of fluid inclusions in order to reconstruct past hydrothermal evolution is strongly dependent onto pressure estimation since significant temperature corrections may be needed in the larderello case failing to take in account the pressure effects would have led to seriously misleading conclusions

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

geochronology and isotope transport systematics in a subsurface granite from the larderello travale geothermal system italy abstract a monzogranite here referred to as the montieri pluton was encountered along two depth profiles from ca 2 to 4 km depth in wells of the larderello travale geothermal system italy we obtained high resolution petrographic and microchemical data on seven fresh samples of the montieri pluton these data are required to correctly interpret the combined isotope data for the rb sr and k ar systems biotite and k feldspar from all samples were analyzed by rb sr and 39ar 40ar rb sr analyses gave extremely variable two point rb sr apparent ages and sr initials indicating that biotite did not equilibrate with feldspar bulk dissolution of k feldspars does not define a rb sr alignment that could have proved a common origin of all k feldspar samples from a single magma batch the leachable fractions of k feldspar separates have much more homogeneous 87sr 86sr ratios around 0 712 which may reflect the isotopic composition of a late stage circulating fluid because of the anatectic origin of the granites from the larderello travale geothermal system it is likely that every k feldspar separate contains at least three isotopically distinct feldspar generations relics of the hercynian gneiss basement pliocene magmatic minerals and hydrothermal retrogression products such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses the 39ar 40ar age spectra of the biotite samples show some internal discordance because of deviations from stoichiometry biotite discordance can be attributed both to chlorite intergrowths as predicted from the active fluid circulation and to the presence of multiple biotite generations such as have already been documented from other micas from the larderello travale geothermal system and as indeed confirmed by high resolution petrography total k ar biotite ages on cluster around 3 ma and broadly agree with a rb sr age obtained by regressing only the biotite analyses this suggests that the montieri pluton whose known volume is at least 25 km3 was emplaced at that time the k feldspar spectra are strongly discordant no step age approaches the zero age which would be predicted by ar diffusivity modelling the extraneous ar in the feldspars is not excess ar but inherited ar instead ar inheritance in the feldspar separates correlates with inheritance of sr arrhenius trajectories of the apparent ar diffusivities for all seven feldspars are astonishingly identical despite the heterochemism amongst samples however the apparent diffusivity calculated from the step heating experiments led to irreproducible models for the thermal history along the depth profile as well as time scales shorter than the historical record this indicates that ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions

dynamics and structure of a stirred partially molten ultralow velocity zone abstract the seismic properties of thin 5 40 km thickness patches exhibiting low seismic velocities termed ultralow velocity zones or ulvz just above the core mantle boundary cmb might be explained by the presence of partially molten rock where a liquid phase occupies interstices within a skeletal network of solid grains however a key problem with this explanation is that in the absence of improbably strong surface tension effects partial melt is expected to drain by percolation over geological time scales to form a dense melt rich layer at the cmb with physical properties that are inconsistent with seismic and other geophysical constraints here we consider whether stirring within ulvz driven by viscous coupling to convective motions in the overlying mantle can inhibit the production of such stratification and maintain a partially molten region with a structure and constitution comparable to what is inferred seismically we use two dimensional numerical simulations of the response of a melt solid mixture to stirring imposed from above and scaling analysis to identify conditions leading to melt separation retention and drainage over a broad range of parameters we find that melt migration at plausible ulvz conditions is governed predominantly by dynamic pressure gradients arising from the viscous deformation related to mantle stirring rather than by the buoyancy effects driving melt percolation in particular dense melt that would otherwise drain downward and accumulate at the cmb is expected to remain in suspension as a result of the stirring driven within ulvz in addition our model predicts that partially molten ulvz patches will be characterized by a positive gradient in seismic shear velocity i e increasing with depth consistent with seismic inferences and may persist in this state over geological time scales

structure and late cenozoic evolution of the upper lithosphere in southwest tuscany italy abstract geological and geophysical data on southwest tuscany are reviewed in order to define the structure and evolution of the upper lithosphere from the miocene to the quaternary petrologic studies reveal the existence below all of tuscany of hercynian and older polyphased metamorphic rocks and of hercynian granite whose top is an important seismic reflecting horizon the basement is characterized by ne sw trending structures in contrast with the main nw se alpine structures of the uppermost levels the heat flow map shows two broad areas with values higher than 80 mw m2 reaching maximum values of 10 5 and 15 h f u in the geothermal areas which are also characterized by negative bouguer anomalies a landsat study revealed a ne sw band of subcircular structures passing through larderello and coinciding with a regional fault system and a steep rise in the moho petrologic geochemical and radiometric data on the tuscan igneous rocks show that partial melting took place in the tuscan crust at different levels and to varying degrees from the miocene to quaternary producing a continuous alpine granitic layer the known tuscan intrusive bodies and two batholiths below the larderello and mt amiata geothermal fields represent culminations of the alpine granite the rise of the tuscan magmas was closely correlated to a post tortonian tensional tectonics and followed its n e migration tensional tectonics started after the last compressional phase 10 11 ma b p as a consequence of the anticlockwise rotation of italy the opening of the tyrrhenian sea and the swelling of the mantle below southwest tuscany

stable isotope and noble gas isotope compositions of inclusion fluids from larderello geothermal field italy constraints to fluid origin and mixing processes noble gas and n 2 compositions have been determined in paleofluids of core samples from geothermal wells in the larderello area italy the results were coupled with hydrogen and carbon isotope compositions of similar fluids from an independent batch of samples to provide constraints on the origin of the fluids the fluids trapped during early stage hydrothermal circulation are characterized by relatively high 4 0 ar 3 6 ar and n 2 ar ratios 332 4 0 ar 3 6 ar 564 and 248 n 2 ar 4197 respectively and indicate the presence of radiogenic 4 0 ar and excess n 2 when compared to air asw air saturated water the fluid inclusions related to late stage hydrothermal activity show low n 2 ar and 4 0 ar 3 6 ar ratios 309 4 0 ar 3 6 ar 354 and 47 n2 ar 66 respectively similar to present day geothermal fluids the 6d values of the inclusion fluids range from 53 to 85 indicating that the fluids were derived from mixing of water rich components probably from a shallow reservoir similar in composition to present day meteoric waters and fluids from a deeper reservoir the r r a ratios up to 3 2 of the paleofluids and present day fluids suggest a deep he contribution at larderello the 1 3 c values for co 2 trapped during early stage activity 3 8 o to 0 5 o were produced by carbonate country rock volatilisation 1 3 c values as low as 17 were measured in inclusion fluids related to late stage circulation and were likely produced by oxidation of organic matter at shallow depths

geochronology and excess ar geochemistry of the lhotse nup leucogranite nepal himalaya abstract 40ar 39ar dating experiments on several coexisting minerals from two close by leucogranite outcrops near lhotse nup glacier nepal himalaya reveal a complex behaviour four biotite and muscovite ages cluster around 15 5 ma a lower value than literature rb sr ages on splits of the same four micas suggesting a discrepancy with the ideal cooling age sequence observed in the alps a strongly discordant ar ar spectrum on tourmaline does not allow a chronological interpretation a potassium feldspar shows a slow cooling staircase spectrum with a superimposed saddle diagnostic of excess ar hf leaching removed excess ar but caused great perturbations to the minimum step ages isochron plots and the release of reactor produced ar isotopes the present data require that the currently fashionable interpretations of feldspar systematics be radically changed the main chronological conclusions rely on the eight mica ages their decrease from 18 2 to 15 3 ma dates the cooling of the lhotse nup leucogranite

dating fault generated pseudotachylytes comparison of 40ar 39ar stepwise heating laser ablation and rb sr microsampling analyses three different geochronological techniques stepwise heating laser ablation 40ar 39ar rb sr microsampling have been evaluated for dating fault generated pseudotachylytes sampled along the periadriatic fault system paf of the alps because pseudotachylytes are whole rock systems composed of melt clast and alteration phases chemical control from both ar isotopes cl k ca k ratios and empa analyses is crucial for their discrimination when applied to stepwise heating 40ar 39ar analyses this approach yields accurate melt related ages even for complex age spectra the spatial resolution of laser ablation 40ar 39ar analyses is capable of contrasting melt clast and alteration phases in situ provided the clasts are not too fine grained the latter of which results in integrated mixed ages without geological information elevated cl k and ca k ratios were found to be an invaluable indicator for the presence of clast admixture or inherited 40ar due to incomplete isotopic resetting during frictional melting rb sr microsampling dating did not furnish geologically meaningful ages on the basis of isotopic disequilibria among pseudotachylyte matrix phases and independent rb sr microsampling dating of cogenetic ultra mylonites the concordant 40ar 39ar pseudotachylyte ages are interpreted as formation ages the investigated pseudotachylytes altogether reveal a cretaceous to miocene history for the entire paf consistent with independent geological evidence individual faults however consistently reveal narrower intervals of enhanced activity lasting a few million years electronic supplementary material to this paper can be obtained by using the springer link server at http dx doi org 10 1008 s00410 002 0381 6

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

multipath ar transport in k feldspar deduced from isothermal heating experiments abstract the hypothesis underlying k feldspar thermochronometry that a single energy fickian diffusion is operating at all temperatures has been investigated three isothermal heating experiments on a granitic microcline show that a discrete distribution of domain sizes is unable to explain experimental data a continuous distribution of domain sizes also fails as its temperature dependence is incorrect the most likely explanation is that fast pathway diffusion is superimposed on the variety of diffusion mechanisms predicted by kinetic theory this prevents extrapolation of laboratory experiments to geological conditions geological models based on k feldspar thermochronometry must be seriously re examined and limited to non isotopic constraints

rb sr and k ar ages on minerals at temperatures of 300 400 c from deep wells in the larderello geothermal field italy rb sr and k ar ages have been obtained on six biotites two muscovites and one hornblende from samples of micaschist gneiss and amphibolite of lower paleozoic to precambrian age at a depth exceeding 2 000 m in basement rocks of the larderello travale geothermal region most of the data cluster in the range 2 5 3 7 ma revealing the existence of a pliocene thermal event to which the origin of the field may be attributed the resulting duration of the larderello geothermal field is unexpectedly long in the basement levels of the two wells examined unstabilized minimum temperatures of 290 and 380 c were measured all the biotites show almost complete 40ar and 87sr retention at the measured well temperatures petrologic evidence stilpnomelane stability and experimental data activation energies and diffusion coefficients also favour a closure temperature above 400 c for rb sr and k ar in biotites in agreement with recent direct experimental determinations