precise ages of the reunion event and huckleberry ridge excursion episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core the reunion event is one of the earliest recognized periods of normal polarity within the reversed matuyama chron named for the site at which it was first discovered on reunion island it has since purportedly been found globally in both volcanic rocks and sediments and thus has become a key chronostratigraphic marker however geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration but also the number of reunion events new 40ar 39ar ages from eight reunion island lavas in three distinct sections are indistinguishable from one another thereby placing the event at 2 200 0 007 0 010 ma2 200 0 007 0 010 ma 2 2 analytical total uncertainty note this format is used throughout the paper the paleomagnetic behavior recorded at two of the island sites shows that the opposite normal polarity was reached and sustained for a period during which several lava flows were erupted whether this can be classified as a very short subchron bounded by a rapid set of back to back reversals or as a special case of a geomagnetic excursion is unclear hence we choose to continue labeling the dynamo activity recorded by these reunion island lavas as an event this event preceded a 38 kyr 38 kyr period of normal polarity that we name the feni subchron after its locality of discovery at odp site 981 the feni subchron was succeeded by the huckleberry ridge excursion for which 40ar 39ar sanidine and u pb zircon ages of 2 077 0 001 0 003 ma2 077 0 001 0 003 ma and 2 084 0 012 0 013 ma2 084 0 012 0 013 ma respectively from member b of the huckleberry ridge tuff in idaho are in agreement these findings suggest that the full normal polarity recorded on reunion island is a singular brief period of unstable field behavior at the onset of a 125 kyr 125 kyr bundling of dynamo instabilities from 2 20 to 2 07 ma disturbances to the axial dipole component of earth s magnetic field during this period and by analogy similar periods of temporally clustered excursions during the early and late portions of the brunhes chron may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner core or core mantle boundaries a proposition that might be tested by future numerical dynamo simulations

monazite dating of granitic gneisses and leucogranites from the kerala khondalite belt southern india implications for late proterozoic crustal evolution in east gondwana two stages of granitic magmatism occurred during the pan african evolution of the kerala khondalite belt kkb in southern india granitic gneisses were derived from porphyritic granites which intruded prior to the main stage of deformation and peak metamorphism subsequently leucogranites and leucotonalites formed during fluid absent melting and intruded the gneiss sequences monazites from granitic gneisses leucogranites and a leucotonalite were investigated by conventional u pb and electron microprobe dating in order to distinguish the different stages of magma emplacement u pb monazite dating yielded a wide range of ages between 590 520 ma which are interpreted to date high grade metamorphism rather than magma emplacement the results of this study indicate that the kkb experienced protracted heating 50 ma at temperatures above 750 800 c during the pan african orogeny the tectonometamorphic evolution of the study area is comparable to southern madagascar which underwent a similar sequence of events earlier than the kkb the results of this study further substantiate previous assertions that the timing of high grade metamorphism in east gondwana shifted from west to east during the late proterozoic

nature of charnockite and closepet granite in the dharwar craton implications for the architecture of the archean crust abstract the architecture of the archean crust is crucial in understanding the geodynamics of cratonization the dharwar craton in the indian peninsula is well known for its continuous change of metamorphic grades from granulite facies in the south to amphibolite facies in the middle and greenschist facies in the north here the genetic relationship of different units is re analyzed to decipher the crustal architecture of the craton the major lithologic unit the granitoids can be subdivided into ttg charnockite closepet granite and anatectic granites migmatites the charnockite near salem has two types of occurrence one is massive garnet free and plutonic crystallized at 2560 ma and the other is gneissic garnet bearing and incipient metamorphosed at 2490 ma both types are compositionally close to tonalite granodiorite and generally show positive eu anomalies a large proportion of zircons of both types show positive hft values varying from 3 to 9 indicating a relatively juvenile parent 2900 ma crust at medium pressure in the middle crustal level with significant mantle contribution the genetic relationship of different units the spatial age difference of greenstone belts and the spatial variation of metamorphic grade in the dharwar craton suggest a trinity of archean crustal architecture sandwich structure with 2700 2500 ma dominated uppercrust 3400 2900 ma dominated middle crust and 2560 to 2500 ma anatectic lower crust this sandwich structure possibly indicates vertical growth of the crust in the archean the whole craton was tilted at 2500 ma with the southern end uplifted the most by an intra cratonic process and then it was shaped by later orogenic events resulting in the formation of the dominantly n s trending structures in the basement

implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

kinematics strain pattern and geochronology of the salem attur shear zone tectonic implications for the multiple sheared salem namakkal blocks of the southern granulite terrane india abstract structural mapping strain analysis and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the salem attur shear zone in the southern granulite terrane of south india the salem namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss charnockite and mafic granulite and had undergone multiple phases of magmatism spanning over a period of 3 2 0 5 ga the rocks were deformed by four phases of deformation d1 d4 the d1 deformation was characterized by isoclinal and recumbent ne sw trending f1 fold with a pervasive subhorizontal axial planar granulitic fabric s1 and associated quartzofeldspathic leucosomes granulite metamorphism was dated at ca 2 5 2 3 ga the f1 fold and s1 fabric were coaxially refolded by tight to isoclinal upright to steeply inclined ne sw trending f2 folds during d2 deformation the d2 deformation was associated with f2 axial planar shear zones crenulations and leucosomes s2 fabric large scale d2 shear zones characterized by high temperature ductile shear fabric with a vertical flow host syntectonic syenite pluton which was dated at ca 2 5 2 4 ga a p t condition of 7 kb 600 c was inferred for the d2 deformation the d3 deformation was characterized by nw se to e w trending f3 folds and the salem attur shear zone the shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down dip stretching lineation defined by quartz biotite and hornblende minerals and dated at ca 2 0 ga it indicated n nne vergence of thrusting with the mean kinematic vorticity number wm as 0 7 suggesting general simple shear strain with 50 pure shear component the d4 deformation was manifested as nne ssw striking strike slip faults and nw se striking extensional normal faults pseudotachylite veins having an age of 1 9 ga injected during strike slip faulting and granite pegmatite veins showing age of 0 8 0 5 ga intruded during normal faulting the salem namakkal blocks thus recorded a long lived shearing history we suggest that the salem attur shear zone and other shear zones such as palghat cauvery moyar bhavani karur kambam painavu trichur and achankovil shear zones were paleoproterozoic intraterrane shear zones which were overprinted by meso neoproterozoic cambrian ductile and brittle deformations

experimental zircon melt and zircon garnet trace element partitioning and implications for the geochronology of crustal rocks garnet is the most commonly used mineral in thermobarometry whereas zircon is the most robust chronometer to date high grade metamorphic rocks to provide a basis for correlation of zircon and garnet growth we determined experimentally the trace element partitioningbetweenzircon ahydrousgraniticmeltandgarnetat20kbarand800 1000 cforp y rareearthelements ree zr hf th andu inrespecttomelt zirconpreferentiallyincorporatesallinvestigatedelementsapartfromreewithatomicnumberlowerthansm at this pressure and in the chosen composition the distribution coefficient between zircon and melt d zrn melt for ree increases with increasingatomicnumberofthereeandwithdecreasingtemperature dyb is 20at1000 c butmorethananorderofmagnitude higher at800 c the solubilityofzrinhydrousgraniticmelts buffered by zirconisabouta factoroftwo lower at20kbar than what has been previously established for mid crustal pressures large garnet produced in the experiments allowed determination of garnet melt trace element partitioning d grt melt at temperatures of 800 1000 c conditions relevant for partial melting of crustal rocks there is a systematic increase indree with decreasing temperature zircon contains significantly more heavy ree than garnet at temperatures of 800 850 c zircon garnetpartitioncoefficientsofheavy reedecreasewithincreasingtemperaturefromdlu of12at800 cto 1 4at1000 c middle reepartitioningisclosetounityforthewholeinvestigatedtemperaturerange differentempiricallydetermined zircon garnet partition coefficients from granulites and ultra high temperature granulites can potentially be explained by the experimentallydeterminedchangeofpartitioningasafunctionoftemperature thesedatacanassistinestablishingequilibriumbetween garnetandzirconzonesinnaturalrocks andintheconstructionofdetailedpressure temperature timepathsinhigh grademetamorphic rocks

mechanisms and timescales of generating eruptible rhyolitic magmas at yellowstone caldera from zircon and sanidine geochronology and geochemistry we constrain the physical nature of the magma reservoir and the mechanisms of rhyolite generation at yellowstone caldera via detailed characterization of zircon and sanidine crystals hosted in three rhyolites erupted during the c 170 70 ka central plateau member eruptive episode the most recent post caldera magmatism at yellowstone we present 238 u 230 th crystallization ages and trace element compositions of the interiors and surfaces i e unpolished rims of single zircon crystals from each rhyolite we compare these zircon data with 238 u 230 th crystallization ages of bulk sanidine separates coupled with chemical and isotopic data from single sanidine crystals zircon age and trace element data demonstrate that the magma reservoir that sourced the central plateau member rhyolites was long lived 150 250 kyr and genetically related to the preceding episode of magmatism which occurred c 256 ka the interiors of most zircons in each rhyolite were inherited from unerupted material related to older stages of central plateau member magmatism or the preceding late upper basin member magmatism i e are antecrysts conversely most zircon surfaces crystallized near the time of eruption from their host liquids i e are autocrystic the repeated recycling of zircon interiors from older stages of magmatism demonstrates that sequentially erupted central plateau member rhyolites are genetically related sanidine separates from each rhyolite yield 238 u 230 th crystallization ages at or near the eruption age of their host magmas coeval with the coexisting zircon surfaces but are younger than the coexisting zircon interiors chemical and isotopic data from single sanidine crystals demonstrate that the sanidines in each rhyolite are in equilibrium with their host melts which considered along with their near eruption crystallization ages suggests that nearly all central plateau member sanidines are autocrystic the paucity of antecrystic sanidine crystals relative to antecrystic zircons requires a model in which eruptible rhyolites are generated by extracting melt and zircons from a long lived mush of immobile crystal rich magma in this process the larger sanidine crystals remain trapped in the locked crystal network the extracted melts plus antecrystic zircon amalgamate into a liquid dominated i e eruptible magma body that is maintained as a physically distinct entity relative to the bulk of the long lived crystal mush zircon surfaces and sanidines in each rhyolite crystallize after melt extraction and amalgamation and their ages constrain the residence time of eruptible magmas at yellowstone residence times of the large volume rhyolites 40 70 km 3 are 1 kyr conservatively 6 kyr which suggests that large volumes of rhyolite can be generated rapidly by extracting melt from a crystal mush because the lifespan of the crystal mush that sourced the central plateau member rhyolites is two orders of magnitude longer than the residence time of eruptible magma

age and tectonic evolution of neoproterozoic ductile shear zones in the southern granulite terrain of india with implications for gondwana studies 1 the high grade rocks of the southern granulite terrain sgt of peninsular india are bounded to the north by the archean dharwar craton another high grade terrane the mesoproterozoic eastern ghats occurs to the northeast of the sgt the tectonic relationship between these crustal domains is complex we present new geochronological and structural data that indicate a continuation of the dharwar craton into the southern granulite terrain as far south as a newly identified neoproterozoic shear zone here named the karur kamban painavu trichur shear zone kkptsz south of the kkptsz mesoproterozoic dates of the sgt are similar to those recorded in the eastern ghats and the two domains may have been conterminous thirty three new u pb th single zircon and monazite dates of samples from six structural transects across the regional shear zones indicate that the sgt has experienced at least seven thermo tectonic events at 2 5 ga 2 0 ga 1 6 ga 1 0 ga 800 ma 600 ma and 550 ma and two distinct episodes of metasomatism charnockitization between 2 50 2 53 and between 0 55 0 53 ga deformation along a number of major shear zones in the sgt is neoproterozoic to earliest paleozoic in age with an early phase d2 concentrated between 700 800 ma and a later phase d3 between 550 and 600 ma major charnockitization 530 550 ma post dates d3 and is in turn overprinted by granitization retrogression and uplift between 525 and 480 ma the kkptsz active between 560 and 570 ma is either a terrane boundary or a tectonized decollement between cover and archean basement rocks represented by predominantly paragneisses to the south and orthogneisses to the north respectively other regional neoproterozoic shear zones do not appear to separate allochthonous terranes as previously suggested on the basis of nd model ages and rb sr biotite whole rock dates the neoproterozoic cambrian tectonothermal history of the sgt and eastern ghats is similar to that recorded in parts of madagascar east africa and antarctica and is used to reconstruct parts of central gondwana here named the deccan continent with more robust confidence

crustal evolution in south india constraints from nd isotopes nd isotope analyses from a range of lithologies including charnockites metasediments and granitoids that comprise the high grade terrain of south india define contrasting age provinces north of the palghat cauvery shear zone the karnataka craton and the granulite blocks of nilgiri and madras are characterized by model nd ages from 3 4 to 2 4 ga with significant crustal growth during the late archean south of the palghat cauvery shear zone a pan african granulite facies event resulted in sm nd fractionation as observed in several incipient charnockite localities model nd age calculations that allow for sm nd fractionation at 550 ma yield ages in the range of 2 9 to 1 3 ga this range compares closely with published model nd ages from the high grade terrain of sri lanka mid proterozoic crustal growth 1 1 ga observed in the vijayan complex of sri lanka may have provided source material for metasediments characterized by young model nd ages in southern peninsular india the palghat cauvery shear z

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

density and lithospheric strength models of the yellowstone snake river plain volcanic system from gravity and heat flow data abstract the structure and composition of the yellowstone snake river plain ysrp volcanic system were analyzed using gravity data taken at over 30 000 stations in the ysrp and surrounding region additional constraints were provided by tomographic seismic velocity models new heat flow and temperature information gps derived strain rates earthquake locations and chemical analyses of volcanic rocks p wave velocity models and velocity density data based on petrologic information were used to constrain three dimensional density models rheology and strength properties were calculated at selected locations and compared to earthquake focal depths results of this study suggest that the lower crust of the snake river plain srp has been thickened by the addition of an underplated layer composed primarily of clinopyroxene having a density of 3200 kg m3 a mid crustal high density sill is composed of a series of gabbroic lenses inter fingering with the granitic upper crust this geometry yields a bulk composition comparable to diorite and a density of 2900 kg m3 the mid crustal sill varies from 4 to 11 km in thickness resulting in a series of se nw trending gravity highs observed along the axis of the srp the mid crustal sill extends up to 20 km southeast of the srp volcanic field and causes asymmetry of the gravity field the yellowstone plateau volcanic field density model reveals low density partial melt 10 km beneath the caldera that shallows under the northeastern caldera and continues laterally 20 km north of the caldera boundary and notably increases the previously estimated size of the magma reservoir by 20 the caldera melt body has a density of 2520 kg m3 and a significantly lower value of 2470 kg m3 for the northeastern caldera melt body southwest of yellowstone the crustal section occupied by the mid crustal sill in the srp and the partial melt in yellowstone constitute a transition area between the active yellowstone magma system and the now volcanically quiescent srp with a density of 2820 kg m3 strength models show that the crust of the ysrp becomes progressively stronger and cooler with increasing distance from yellowstone and tectonic earthquakes within the yellowstone caldera are unlikely to nucleate below 4 6 km depth thus limiting the maximum magnitude of earthquakes to m 6 5

geochemistry of high grade supracrustal rocks from the kerala khondalite belt and adjacent massif charnockites south india abstract the kerala khondalite belt kkb comprises a suite of supracrustal rocks of probable proterozoic age that was metamorphosed to the granulite facies the kkb consists of an interlayered sequence of 1 garnet biotite orthopyroxene graphite gneisses 2 khondalites graphite garnet biotite sillimanite cordierite gneisses and 3 cordierite gneisses garnet biotite cordierite orthopyroxene as well as less abundant mafic granulites calc silicates and quartzites petrologic and geochemical data suggest that these rocks were originally arkosic and argillaceous sediments possibly accompanied by felsic volcanic rocks along with minor intercalations of basalts carbonates and quartz rich sandstones the ree patterns of the metasediments are typically lree enriched with large negative eu anomalies indicating a continental provenance the major and trace element compositions of some of the arkosic rocks of the kkb resemble those of the massif charnockites that bound the kkb to the north which suggests that the massif charnockites may have been the source of some of the kkb sediments modern day analogues of the pelite arkose lithologic association observed in the kkb occur in intracratonic rifts and rifted continental margins this association is clearly distinct from the graywacke mafic volcanic association found in the southern indian greenstone belts and many other lower grade archean sedimentary sequences the kkb supracrustals show both lithologic and chemical similarities to many proterozoic supracrustal successions

rapid magma evolution constrained by zircon petrochronology and 40ar 39ar sanidine ages for the huckleberry ridge tuff yellowstone usa understanding the time scales of magmatic differentiation storage and eruption of large volume silicic magmas is a primary goal of igneous petrology within the huckleberry ridge tuff hrt idaho usa representing the earliest and largest caldera forming eruption associated with yellowstone volcanic activity zircon morphological zoning patterns coupled to strongly correlated changes in ti in zircon thermometry and trace element indicators of progressive differentiation provide a proxy record for the evolution of the hrt member b magma body tandem in situ and isotope dilution u pb dating of single zircon crystals demonstrates an absence of pre pleistocene xenocrysts but reveals the presence of antecrysts recycled from pre caldera rhyolites in the hrt magma the petrochronologic interpretation of autocrystic zircon thermal chemical and temporal characteristics suggests that hrt member b differentiated over 10 k y prior to eruption at 2 0794 0 0046 ma as defined by new astronomically calibrated single crystal total fusion 40 ar 39 ar sanidine analyses this refined eruption age demonstrates that the transitional polarity preserved by hrt member b does not record the reunion subchron but rather a separate younger geomagnetic event our novel approach places the thermal and chemical regime of silicic magmas within a temporal context and demonstrates the rapid evolution of a large volume of silicic magma

an experimental study of trace element partitioning between zircon and melt as a function of oxygen fugacity abstract the partitioning of p sc ti sr y nb the rare earth elements ree hf ta th and u between zircon and a synthetic andesitic melt was determined as a function of oxygen fugacity f o 2 over a range of fourteen log units from qfm 8 to qfm 6 where qfm is the quartz fayalite magnetite oxygen buffer at 1300 c and 1 atm using sims and la icp ms the partition coefficients of ce and eu were found to vary systematically with f o 2 relative to those of the other ree producing ce and eu anomalies similar to those that are characteristic of natural igneous zircon these anomalies coexist at terrestrial f o 2 s and were used to quantify eu 3 eu 2 eu 3 and ce 4 ce 3 ce 4 in the melt the partition coefficients of the redox invariant trivalent cations are in excellent agreement with lattice strain theory the values of d ree are related to those determined for natural samples by the expected dependence on temperature the incorporation of ree 3 in zircon was independent of the presence of p d u d th was found to vary systematically with f o 2 indicating the presence of u 4 u 5 and u 6 in the melt at terrestrial conditions d ti was independent of f o 2 despite an expectation of significant ti 3 in the most reduced experiments

episodic growth and homogenization of plutonic roots in arc volcanoes from combined u th and u th he zircon dating abstract tracing the fate of unerupted magma is challenging because plutonic roots of young volcanoes are largely inaccessible here we develop the use of zircon age spectra to determine crystal provenance and source rocks for volcanic products in analogy to detrital crystals in sediments u th zircon crystallization ages for the soufriere volcanic complex saint lucia lesser antilles frequently predate their eruption as determined from combined u th and u th he zircon dating the oldest dated eruptions are 273 15 ka and 264 8 ka 1 uncertainty for morne bonin dacite and bellevue pumice deposit respectively the most recent eruptions formed morphologically pristine domes in the center of the qualibou depression belfond 13 6 0 4 ka terre blanche 15 3 0 4 ka u th u pb zircon crystallization ages determined for crystal rims and interiors range between near eruption ages to 600 ka older xenocrysts are absent zircon crystallization age distributions are complex yet systematic crystal rim ages in the most recently erupted volcanic rocks match those of co erupted plutonic inclusions whereas crystal interiors are equivalent to the cumulative distribution of zircon ages from older eruptions this is evidence that silicic lava domes and pyroclastic flows share a common source that is located underneath the qualibou depression where the intrusive roots of this long lived arc volcanic system became homogenized through thermal and mechanical reprocessing of individual batches of unerupted magma from earlier volcanic episodes within timescales of

rates of magma differentiation and emplacement in a ballooning pluton recorded by u pb tims tea adamello batholith italy abstract geochemical structural field and geochronological data have been used to arrive at very different models for the construction of upper crustal batholiths models for pulsed intrusion of small magma batches over long timescales 1 ma versus transfer of larger magma bodies on shorter timescales predict a different thermal metamorphic and rheological state of the crust highlighting the importance of robust time constraints this study focuses on a well characterized upper crustal intrusion the 15 km2 lago della vacca complex lvc adamello batholith n italy previous studies used structural and petrologic data to argue that the lvc was emplaced through pulsed magma injection and in situ expansion or ballooning of a short lived 105 yr magma chamber we test this model using a dense sampling strategy and high precision id tims u pb geochronology of zircon and titanite combined with hafnium isotope and trace element analyses of the same volume of dated mineral u pb tims tea these data show that the marginal mafic pulses of magma crystallized zircon with primitive hf isotopes and negligible eu anomalies during fractional crystallization and ascent through the crust on 10 30 ka timescales subsequent more felsic pulses yield individual zircon dates spanning as much as 200 ka within single handsamples and restrict the total construction time of the lvc to

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

zircon growth and recycling during the assembly of large composite arc plutons abstract concordant u pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth advances in zircon tims analysis and geochronological studies by sims on zircon from young volcanic rocks along with evolving views of the evolution of magmatic systems makes it necessary to revaluate this interpretation assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton distinctions must be made between true inherited zircon xenocrystic zircon derived from host rocks during magma ascent and or emplacement and multi stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons these complications are exemplified by recent high precision u pb zircon age dating from the cretaceous tuolumne and mt stuart batholiths which clearly show that the timeframe of pluton assembly was long 5 ma for mt stuart and 8 ma for tuolumne zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon several 10 5 yr up to 2 10 6 yr and assignment of statistically valid rock solidification ages is not possible from these data low zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages and is compatible with progressive growth of a large long lived crystal mush body because eruption of magma requires low to moderate crystal fraction and high temperatures relative to stored crystal mushes zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions

the application of single zircon evaporation and model nd ages to the interpretation of polymetamorphic terrains an example from the proterozoic mobile belt of south india the technique of single zircon dating from the thermal evaporation of 207pb 206pb kober 1986 1987 provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages in contrast nd model ages may provide a measure of the period of crustal residency for the sample or its protolith these two techniques have been combined to elucidate the tectonic history of the proterozoic mobile belt of southern india exposed south of the palghat cauvery shear zone that marks the southern boundary of the archaean craton of karnataka the two main tectonic units of this mobile belt comprise the madurai and trivandrum blocks both of which are characterised by massive charnockite uplands and low lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the proterozoic evidence for early palaeoproterozoic magmatism is restricted to the madurai block where single zircon evaporation ages from a metagranite 2436 4 ma are similar to model nd ages from a range of lithologies suggesting crustal growth at that time the trivandrum block to the south of the achankovil shear zone is comprised of the kerala khondalite belt the nagercoil charnockites and the achankovil metasediments single zircon evaporation ages together with conventional zircon and garnet chronometry suggest that all three units underwent upper amphibolite facies metamorphism at 1800 ma an event unrecorded in the metagranite from the madurai block this implies that the madurai and trivandrum blocks represent distinct terrains throughout the palaeoproterozoic model nd ages from the achankovil metasediments are much younger 1500 1200 ma than those from the adjacent kerala khondalite belt and madurai blocks 3000 2100 ma but there is no evidence for zircon growth in these metasediments during the mesoproterozoic hence the comparatively young model nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth zircon overgrowths from the madurai block 547 17 ma and achankovil metasediments 530 21 ma suggest that all tectonic units of the proterozoic mobile belt of south india shared the same metamorphic history from the early palaeozoic this event has been recognised in the basement lithologies of sri lanka and east antarctica confirming that the constituent terrains of east gondwana had assembled by this time

mushy magma beneath yellowstone a recent prospective on the yellowstone caldera discounts its explosive potential based on inferences from tomographic studies which suggests a high degree of crystallization of the underlying magma body in this study we show that many of the first teleseismic p wave arrivals observed at seismic stations on the edge of the caldera did not travel through the magma body but have taken longer but faster paths around the edge after applying a number of waveform modeling tools we obtain much lower seismic velocities than previous studies 2 3 km sec v p and 1 1 km sec v s we estimate the physical state of the magma body by assuming a fluid saturated porous material consisting of granite and a mixture of rhyolite melt and water and co 2 at a temperature of 800 c and pressure at 5 km 0 1 gpa we found that this relatively shallow magma body has a volume of over 4 300 km 3 and is about 32 melt saturated with about 8 water plus co 2 by volume

tectonics of the neoproterozoic southern granulite terrain south india geological correlations between south india and adjacent gondwana fragments are hampered by the lack of a coherent structural framework for india s southern granulite terrain sgt this paper is an attempt at delineating its large scale geometry and neoproterozoic kinematics and tectonics the basic regional pattern is that of a high grade gneissic layering produced during several deformation stages grouped as d1 which became refolded during two regionally separated phases of upright folding d2a nnw trending d2b ne trending and reworked in e to ese trending zones of increased strain d3 of these the palghat cauvery shear zone pcsz which marks the boundary between archaean terrains in the north and proterozoic belts in the south may represent a major crustal shear zone whereas the achankovil zone az is better considered as a high strain zone characterized by flattening with a reverse component d1 structures developed close to the peak of pan african high grade metamorphism d2a and d2b occurred on the retrograde path partly at supersolidus and partly at subsolidus conditions d3 high strain zones show subsolidus textures in view of the general clockwise nature with a clear decompressional component of all published p t paths we propose that d2 d3 occurred during partial exhumation of the terrain correlations with madagascar are weakened as the az is no longer considered a major crustal shear zone whereas correlations with sri lanka are strengthened by the presence of a similar syntactical bend possibly produced by an indentor

experimental measurements of zircon melt trace element partition coefficients abstract zircon was grown from trace element doped hydrous peralkaline rhyolite melts with buffered oxygen fugacities in cold seal experiments at 0 1 and 0 2 gpa and 800 c and piston cylinder experiments at 1 5 gpa and 900 1300 c zircon and glass were present in all run products and small monazite crystals were present in eight of the 12 experiments average diameters of zircon crystals ranged from 5 to 20 m at 800 c to 30 50 m at 1300 c zircon crystals have thin rims and adjacent glass has a narrow 1 m thick compositional boundary layer concentrations obtained through in situ analysis of cores of run product zircon crystals and melt pools were used to calculate trace element partition coefficients dzircon melt for p sc ti v y la ce pr nd eu gd ho yb lu hf th and u in most cases lu was the most d 12 105 and la the least 0 06 0 95 compatible elements d values from this study fall within the range of previously measured values for rare earth elements ree however d values measured experimentally show less fractionation than those recently measured using natural phenocryst matrix pairs for example dlu dla measured experimentally in this study range between 27 and 206 compared to a value of 706 522 for a natural zircon dacite pair sano y terada k and fukuoka t 2002 high mass resolution ion microprobe analysis of rare earth elements in silicate glass apatite and zircon lack of matrix dependency chem geol 184 217 230 although d values from this study show good agreement with the lattice strain model d values from natural phenocryst matrix pairs combined with measured zircon compositions better reproduce host rock magma compositions of igneous rocks they also yield more reasonable estimates of magma compositions when combined with compositions of out of context zircons for example compositions of the hadean detrital zircons from jack hills australia yield lree enriched magmas when combined with d values from phenocryst matrix pairs yields but yield lree depleted magmas when experimentally determined d values are used we infer that experimentally measured dzircon melt values represent disequilibrium partitioning resulting from rapid zircon growth during short laboratory timescales rapid growth causes development of observed diffusive boundary layers in the melt adjacent to zircon crystals d values from phenocryst matrix pairs are therefore recommended for petrogenetic modeling

laacher see revisited high spatial resolution zircon dating indicates rapid formation of a zoned magma chamber high spatial resolution 230 th 238 u disequilibrium dating reveals rapid zircon crystallization prior to the 12 900 yr b p laacher see germany phonolite eruption zircons from lower laacher see tephra llst and syenitic subvolcanic nodules share ree and low u th characteristics and define an isochron age of 17 1 1 3 ka 1 zircons also show higher initial 230 th 232 th 0 and oxygen isotopic disequilibrium compared to their host phonolite thus llst zircons shortly predate the eruption and yet are not strictly part of the phonolite crystallization sequence based on their similarity to zircons in subvolcanic nodules they are instead interpreted to be scavenged from a precursor syenite intrusion which implies short a few k y filling and differentiation timescales for the laacher see magma chamber thorium and oxygen isotopes in zircon as well as evidence for abundant crustal xenocrysts further indicate extensive crustal contamination which allows for considerably faster basanite phonolite differentiation than the 100 k y time span previously estimated from bulk u series data

dehydration melting phenomena in leptynitic gneisses and the generation of leucogranites a case study from the kerala khondalite belt southern india

pbpb dating of apatite by a stepwise dissolution technique abstract apatites from different geologic settings have been dated by a new sequential dissolution method the procedure is based on stepwise exposure of apatite to weak intermediate acids a pegmatite sample from the kerala khondalite belt in southern india yielded an age of 509 25 ma apatites from an enderbite sample from the northern marginal zone of the limpopo belt in southern africa yielded an age of 1970 26 ma which is in agreement with the high grade metamorphism at around 2 0 ga reported from the triangle shear zone and amphibolite facies metamorphism in the transition zone apatites from an amphibolite facies gneiss from the transition zone south of the nmz yielded an age of 1878 18 ma this age is somewhat younger than a sphene age on the same rock of 1971 17 ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase the sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic pb ratios because the spread can be significantly increased and no correction for common lead is needed this study further demonstrates that apatites from magmatic and high grade metamorphic rocks yield ages very close to those of monazite zircon or garnet unless the rock subsequently interacted with a fluid phase

evolution of the continental crust in the kerala khondalite belt southernmost india evidence from nd isotope mapping u pb and rb sr geochronology in order to decipher crustal genesis and evolution as well as to refine the metamorphic history of the kerala khondalite belt kkb southernmost india we have applied u pb monazite rb sr biotite feldspar dating and nd isotope geochemistry the kkb belonged to an eastern gondwana mobile belt that underwent ultra high temperature metamorphism at pan african times it is dominated by nd model ages ranging between 2 0 and 3 0 ga except at its northernmost boundary the achankovil unit au where middle proterozoic nd model ages were recognised 1 3 1 6 ga the new data show that these ages are not restricted to cordierite bearing metasediments alone but can be found in other lithologies of the achankovil unit as well peak metamorphism occurred between 590 and 550 ma as recorded by u pb dating of monazite from charnockites and garnet biotite gneisses rb sr ages of minerals in pegmatite intrusions record cooling to ca 400 500 c by 490 470 ma based on a systematic regional sampling this contribution confirms and refines former models based on smaller datasets the major issue is that the kerala khondalite belt must have undergone an extremely slow cooling history 3 6 c ma comparable to the one described for sri lanka and madagascar still such ultra high temperatures associated to slow cooling processes cannot be satisfactorily fit into a common tectonic context 2004 published by elsevier b v

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

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

40ar39ar geochronology of rhyolites erupted following collapse of the yellowstone caldera yellowstone plateau volcanic field implications for crustal contamination abstract single crystal laser probe 40 ar 39 ar dating of 133 grains of sanidine and plagioclase has enabled us to resolve the eruption ages of the upper basin member rhyolites the lava flows and related tuffs that erupted within the yellowstone caldera shortly after its collapse 630 ky ago on eruption of the lava creek tuff two lavas and a tuff that erupted from the eastern ring fracture zone yield an eruptive age of 481 8 ka whereas two flows from the western ring fracture zone yield eruptive ages of 516 7 and 198 8 ka most of the units contain old xenocrysts explaining why previous attempts at dating these earliest post caldera units by the conventional k ar method yielded poorly resolved and in some cases anomalous ages the tuff shows the most severe contamination grains from a single pumice lapilli in the tuff show as large an age range as those from bulk vitrophyre indicating that the xenocrysts were incorporated in the magma prior to its near surface explosive fragmentation diffusion calculations indicate that the xenocrysts could not have remained in the magma for more than a few years without degassing and giving ages indistinguishable from the phenocrysts thus the contamination represented by the xenocrysts probably occurred during fracturing and conduit propagation rather than during caldera collapse which took place more than 100 ky earlier the apparent ages of xenocrysts and their compositions as determined by electron microprobe suggest that the eocene absaroka volcanics are the main contaminant with a single xenocryst probably coming from precambrian basement rocks most of the xenocrysts are difficult to distinguish optically or chemically from feldspar phenocrysts illustrating the necessity of single crystal to date many young volcanic rocks accurately

gas driven filter pressing in magmas most silicic and some mafic magmas expand via second boiling if they crystallize at depths of about 10 km or less the buildup of gas pressure due to second boiling can be relieved by expulsion of melt out of the region of crystallization and this process of gas driven filter pressing assists the crystallization differentiation of magmas for gas driven filter pressing to be effective the region of crystallization must inflate slowly relative to buildup of pressure and expulsion of melt these conditions are satisfied in undercooled magmatic inclusions and in thin sheets of primitive magma underplating cooler magma reservoirs gas driven filter pressing thereby adds fractionated melt to magma bodies gas driven filter pressing is probably the dominant process by which highly evolved melts segregate from crystal mush to form aplitic dikes in granitic plutons this process could also account for the production of voluminous crystal poor rhyolites

alkali granites with pan african affinities from kerala s india in the kerala region of the south indian shield alkali granite plutons ranging in age from 550 to 740 ma intrude precambrian charnockites and migmatites all of them are spatially related to regional fault lineaments their geochemistry is similar to pan african post kinematic plutons from the afro arabian shield they are of granitic to adamellitic composition with high sio 2 k 2 o up to 12 and na 2 o and low cao feo and mgo their major element composition together with a low 87 sr 86 sr i 0 703 from one pluton denote a type affinities however local charnockites have such low rb and radiogenic 87 sr that the granites could have formed by crustal melting their overall trace element characteristics are typical of alkaline granites in general but the details show that most resemble more recent volcanic arc granites and one pluton is typical of a within plate setting the presence of co 2 rich fluid inclusions supports the notion that the enrichment of rare earth an

constraints on the nature of the subvolcanic reservoir at south sister volcano oregon from u series dating combined with sub crystal trace element analysis of plagioclase and zircon abstract we present sub crystal scale 238u 230th zircon ages and 238u 230th 226ra plagioclase ages of bulk mineral separates from the holocene 2 0 2 3 ka eruptions of the rock mesa rm and devil s hills dh rhyolites at south sister volcano oregon we link these age data with sub crystal trace element analyses of zircon and plagioclase to provide insight into the subvolcanic system at south sister as an example of a small volume continental arc volcano our results document the presence of coeval yet physically distinct regions within the magma reservoir and constrain the timescales over which these heterogeneities existed zircons from the rm and dh dominantly record ages from 20 to 80 ka with some grains recording ages 350 ka whereas plagioclase records 230th 226ra ages of 2 3 6 8 ka rm and 4 0 9 6 ka dh 3 and a 238u 230th age of 10 34 ka dh 3 we interpret zircons with ages 50 ka within a heterogeneous magmatic system and coexisted as physically distinct dominantly liquid bodies prior to eruption thus we favor a model where rhyolites are generated in independent batches by accumulation of evolved liquids in a heterogeneous largely crystalline reservoir similarities in crystal age and chemical data to that at other young silicic systems e g mount st helens okataina caldera complex suggest that this model may be more generally applicable to silicic magmas

constraints on crystal storage timescales in mixed magmas uranium series disequilibria in plagioclase from holocene magmas at mount hood oregon abstract uranium series crystal ages interpreted within a textural and geochemical framework can provide insight into crystal storage timescales especially in cases where crystals may derive from multiple sources we report here 230 th 226 ra model ages of two distinct populations of plagioclase from low silica dacites from mount hood oregon a volcano where previous studies show that the compositions of erupted magmas are controlled by magma recharge mixing and incorporation of plagioclase derived from mafic and silicic end member magmas we have measured trace element concentrations and 238 u 230 th 226 ra disequilibria in four plagioclase size fractions from the timberline 1500 a and old maid 215 a eruptive sequences after correction for groundmass and apatite contamination average 230 th 226 ra model ages of large 500 m plagioclase are 4 5 ka timberline and 5 5 ka old maid with ages of cores that are 10 ka in each case indicating that plagioclase derived from silicic magmas crystallized thousands of years before eruptions these model ages are longer than timescales of repose between eruptions indicating that these crystals resided in the sub surface over multiple eruptions likely stored in a silicic crystal mush zone that periodically interacts with mafic recharge magmas remobilizing a fraction of the large plagioclase crystals during each eruptive event after correction for large plagioclase contamination small 226 ra ba relative to equilibrium with liquid proxies groundmass and mafic inclusion leading to 230 th 226 ra model ages that are 230 th 226 ra disequilibria require that the majority of crystals in the separate are young 226 ra ba could potentially be explained by rapid crystallization immediately prior to and or during mixing events consistent with evidence of rapid crystallization of rims rapid crystallization of mafic intrusions may trigger eruption at mount hood by producing a partially crystalline mafic magma capable of mixing with a reheated silicic crystal mush

uranium series accessory crystal dating of magmatic processes complex and protracted crystallization histories over geologic timescales are recorded in accessory minerals e g zircon allanite although magmatic crystallization was traditionally assumed to occur essentially instantaneously for the purposes of interpreting mineral geochronometers with low absolute time resolution for ancient samples it emerged relatively recently that magmatic crystallization can occur over extended durations this discovery arose from applying high spatial resolution accessory mineral dating techniques for uranium series isotopes to young volcanic and cognate plutonic rocks the emerging pattern from these studies is that individual crystals and crystal populations record crystallization episodes lasting from 1 000 to many hundreds of thousands of years accessory mineral dating of volcanic rocks and cognate plutonic xenoliths opens new research avenues for crystal age fingerprinting that correlates pyroclastic deposits lavas and plutonic rocks by using characteristic age dis

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

age intercalibration of 40ar 39ar sanidine and chemically distinct u pb zircon populations from the alder creek rhyolite quaternary geochronology standard abstract we report results from a 40 ar 39 ar sanidine and ca tims 238 u 206 pb zircon dating study of eruption and crystal residence timescales of the alder creek rhyolite acr california extruded during the cobb mountain normal polarity subchron c1r 2n a 40 ar 39 ar acr sanidine date of 1 1850 0 0016 ma 2 external uncertainty determined relative to the astronomically dated a1 tephra sanidine is interpreted as the acr eruption age this age is supported by ca tims u pb zircon dating guided by la icpms trace element analyses titanium in zircon tizr thermometry and cathodoluminescence cl imaging using these data two compositionally distinct zircon populations were revealed pre acr group b zircon exhibit oscillatory zoning large positive ce and negative eu anomalies high incompatible trace element contents tizr temperatures of 650 c 750 c and th u disequilibrium corrected 238 u 206 pb dates of 1 38 1 24 ma these crystals are interpreted as antecrysts inherited from earlier intrusives in the geysers cobb mountain magma source region acr related group a zircon present as discrete grains and overgrowths on group b zircon display less intense cl with diffuse zoning and have less pronounced positive ce and negative eu anomalies lower incompatible trace element contents higher tizr temperatures that range up to 840 c and significantly younger dates the youngest group a dates yield a weighted mean of 1 1978 0 0046 ma 2 including systematic uncertainties that is interpreted as the mean age of zircon crystallization prior to eruption the 13 5 ka offset between the 40 ar 39 ar and 238 u 206 pb dates can be attributed to zircon magma residence time recognition of a young population of acr zircon is consistent with the 40 ar 39 ar eruption age which coincides with the astronomical age estimate for the cobb mountain subchron determined by correlating the oxygen isotope record of the giant piston core md972143 to the la93 1 1 orbital solution on the basis of independent radio isotopic and orbital forcing results we propose the refined age of 1 1850 0 0016 ma 2 external uncertainty for the quaternary acr 40 ar 39 ar sanidine standard

high radiogenic heat production in the kerala khondalite block southern granulite province india in situ radioelemental k u and th analysis and heat production estimates have been made at 59 sites in the kerala khondalite block kkb of the southern granulite province sgp of india together with the in situ analyses on granulites and gneisses previously reported from 28 sites and heat production estimated from the published geochemical analyses on granites and syenites of the kkb the new data set allows good characterization of heat production for the major granulite facies rocks and granitoids of the kkb garnet biotite gneisses are characterized by high levels of th and u with mean values of 60 and 3 ppm respectively khondalites leptynites and charnockites have slightly lower levels of th 23 20 and 22 ppm respectively and u 2 9 2 4 and 0 9 ppm respectively the mean k u th abundances for the granites leucogranites and granitic gneisses ranges from 3 9 to 4 3 2 6 to 4 3 ppm 22 to 50 ppm respectively and for the syenites 4 8 2 ppm and 5 7 ppm mean radiogenic heat production values for garnet biotite gneiss khondalite leptynite and charnockite are 5 5 2 7 2 4 and 2 2 w m 3 respectively for the granites leucogranites granitic gneisses and syenites it is 2 6 3 4 4 6 and 1 4 w m 3 respectively heat production of granulite facies rocks which are the most abundant rocks in kkb correlate well with th but less with u suggesting that variation is caused by th and u bearing accessory minerals such as monazite and zircon the high heat production of the kkb granulites are in contrast to the low heat production of the late archaean granulites of the northern block nb of the sgp which are highly depleted in radioelements and also the granulites of madurai block mb that have higher radioelemental abundances than in the granulites of the nb the high heat production of the kkb granulites could be due to the nature of protoliths and or metasomatism associated with neoproteroic to pan african alkaline magmatic activity represented by alkali granite and syenite carbonatite emplacements and emplacement of pegmatites

in situ u pb ages of zircons from the bishop tuff no evidence for long crystal residence times ion microprobe u pb isotope analyses of zircons from fallout and intercalated ash flow deposits of the early most chemically evolved bishop tuff east central california yield a weighted mean age of 823 11 ka mean square of weighted deviates mswd 0 8 n 22 a similar mean age of 839 36 ka mswd 0 6 n 15 obtained on unpolished zircon rims suggests that the difference between these ages and that of 760 ka for the bishop tuff obtained by recent ar isotope determinations may be largely analytical in origin considerations based only on the effect of 230 th 234 u disequilibria on the u pb ages yield an apparent maximum mean crystal residence time for zircon of 100 k y the zircon residence times are significantly shorter than those inferred from previous estimates for the timing of phenocryst growth and suggest that once evolved the early bishop tuff rhyolite was not stored for long before erupting

zircon reveals protracted magma storage and recycling beneath mount st helens current data and models for mount st helens volcano washington united states suggest relatively rapid transport from magma genesis to eruption with no evidence for protracted storage or recycling of magmas however we show here that complex zircon age populations extending back hundreds of thousands of years from eruption age indicate that magmas regularly stall in the crust cool and crystallize beneath the volcano and are then rejuvenated and incorporated by hotter young magmas on their way to the surface estimated dissolution times suggest that entrained zircon generally resided in rejuvenating magmas for no more than about a century zircon elemental compositions refl ect the increasing infl uence of mafi c input into the system through time recording growth from hotter less evolved magmas tens of thousands of years prior to the appearance of mafi c magmas at the surface or changes in whole rock geochemistry and petrology and providing a new time correlated record of this evolution independent of the eruption history zircon data thus reveal the history of the hidden long lived intrusive portion of the mount st helens system where melt and crystals are stored for as long as hundreds of thousands of years and interact with fresh infl uxes of magmas that traverse the intrusive reservoir before erupting

months between rejuvenation and volcanic eruption at yellowstone caldera wyoming rejuvenation of previously intruded silicic magma is an important process leading to effusive rhyolite which is the most common product of volcanism at calderas with protracted histories of eruption and unrest such as yellowstone caldera wyoming long valley caldera california and valles caldera new mexico in the united states although orders of magnitude smaller in volume than rare caldera forming supereruptions these relatively frequent effusions of rhyolite are comparable to the largest eruptions of the 20th century and pose a considerable volcanic hazard however the physical pathway from rejuvenation to eruption of silicic magma is unclear particularly because the time between reheating of a subvolcanic intrusion and eruption is poorly quantified this study uses nanometer scale trace element diffusion in sanidine crystals to reveal that rejuvenation of a near solidus or subsolidus silicic intrusion occurred in 10 mo or less following a protracted period 220 k y of volcanic repose and resulted in effusion of 3 km3 of high silica rhyolite lava at the onset of yellowstone s last volcanic interval the future renewal of effusive silicic volcanism at yellowstone will likely require a comparable energetic intrusion of magma that remelts the shallow subvolcanic reservoir and generates eruptible rhyolite on month to annual time scales

fluid rock history of granulite facies humite marbles from ambasamudram southern india an extensive humite bearing marble horizon within a supracrustal sequence at ambasamudram southern india was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics at peak metamorphic temperatures of 775 73 c based on calcite graphite carbon isotope thermometry the mineral assemblages suggest layer by layer control of fluid compositions clinohumite calcite bearing assemblages suggest xco2 0 9 at 790 c and local wollastonite scapolite grossular bearing zones formed at xco2 of c 0 3 retrograde reaction textures such as scapolite quartz symplectites after feldspar and calcite and replacement of dolomite diopside or tremolite dolomite after calcite forsterite or calcite clinohumite are indicative of retrogression under high xco2 conditions calcite preserves late proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting isotopic zonation of calcite and graphite grains was likely produced by localized co2 fluid infiltration during retrogression contrary to the widespread occurrence of humite marbles related to retrograde aqueous fluid infiltration the ambasamudram humite marbles record a prograde to peak metamorphic humite formation and retrogression under conditions of low xh2o

the yellowstone hotspot in space and time nd and hf isotopes in silicic magmas abstract over the course of its 16 m y history the yellowstone hotspot has produced silicic magmas exhibiting systematic and often sympathetic variations in isotopic and chemical composition temperature and frequency of eruption nd and hf isotopic ratios vary systematically from initial eruptions at 16 ma contemporaneous with basaltic volcanism in eastern oregon and washington to the present day yellowstone volcanic plateau nd and hf isotopic ratios co vary and span the range of most terrestrial samples reflecting mixing of mantle and crustal sources earliest erupted silicic magmas were hot in excess of 1050 c relatively less evolved and have isotopic ratios within the range of contemporaneous columbia river flood basalts the transit of the hotspot across the lithospheric boundary between the western accreted oceanic terrain and the precambrian craton at 15 ma is marked by shifts in e nd from 4 to 11 and in e hf from 10 to 10 the duration of the transit yields a crustal magma source diameter of 70 km in the interval from 14 to 9 ma e nd systematically increases from 11 to 7 recording a minimum increase in the mantle component from 5 to 30 the mantle component could be twice as great depending upon the isotopic composition of crust and mantle reservoirs in this same interval peak temperatures of 1000 c occurred at 9 ma the last 8 m y are characterized by less frequent eruption of lower temperature 830 900 c and more compositionally evolved magmas

argon geochronology of kilauea s early submarine history submarine alkalic and transitional basalts collected by submersible along kilauea volcano s south flank represent early eruptive products from earth s most active volcano strongly alkalic basalt fragments sampled from volcaniclastic deposits below the mid slope hilina bench yield 40 ar 39 ar ages from 212 38 to 280 20 ka these ages are similar to high precision 234 9 and 239 10 ka phlogopite ages from nephelinite clasts in the same deposits above the mid slope bench two intact alkalic to transitional pillow lava sequences protrude through the younger sediment apron samples collected from a weakly alkalic basalt section yield 138 30 to 166 26 ka ages and others from a transitional basalt section yield 138 115 and 228 114 ka ages the ages are incompatible with previous unspiked k ar studies from samples in deep drill holes along the east rift of kilauea the submarine birth of kilauea volcano is estimated at 300 ka if the weakly alkalic sequence we dated is representative of the volcano as a whole the transition from alkalic to tholeiitic basalt compositions is dated at 150 ka

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

magmatic perturbations in the okataina volcanic complex new zealand at thousand year timescales recorded in single zircon crystals abstract using single crystals to trace the chemical evolution of a magmatic system has long been a goal of igneous petrology crystals utilized for in situ dating hold great potential to link compositional and temporal information to better understand the evolution of a magmatic system if micron scale zoning of trace elements within a single zircon can be directly associated with volcanic events in magmatic systems then new insights into long term maintenance and storage of eruptible magma can be unlocked this study presents new data that directly links the geochemical history recorded in individual zircon crystals with over 50 000 yrs of changing physical and chemical conditions within a magma reservoir trace elements hf and y in zircon have not diffusively equilibrated with their host melt so that they store information about the melt when that zone crystallized thus different stages of growth can be associated with discrete time periods in the magmatic system zircon from the two most recent rhyolite eruptions kaharoa and whakatane within the okataina volcanic complex ovc new zealand have both isotopic age and trace element signatures that correspond with distinct changes in the temperature and phase assemblage of pulses of erupted magma within the caldera system this discrete change is associated with caldera collapse and reflects a change from cold wet source rhyolite to a relatively hotter and drier source rhyolite and back the kaharoa and whakatane eruptions are separated by 4000 yrs and 15 km distance yet the zircon populations record the same distinct thermal and chemical pulse that occurred following caldera collapse suggesting an interconnected magmatic system that houses at least small volumes of rhyolite melt for timescales of 10 3 to 10 5 yrs that is periodically extracted during eruption

nd isotopic evidence for the position of southernmost indian terranes within east gondwana abstract sm nd model ages of orthopyroxene bearing massif charnockites from the cardamom hills massif and adjoining supracrustal rocks from the kerala khondalite belt in southernmost india are used to infer some of the relationships within these rocks and between them and neighboring areas most of these rocks have model ages of 2 1 2 8 ga with most charnockites in the range 2 2 2 6 ga thus 3 0 3 4 ga archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks the similarity of sm nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks a cordierite gneiss representative of a relatively minor lithology in the supracrustal belt has a model age of 1 3 ga the protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks the source material of the gneiss protolith may have been located in the wanni and vijayan complexes of sri lanka the overlap of the model ages of rocks in this area and those in the highland complex of sri lanka supports the notion that these two sets of rocks were joined to each other in gondwana they belong to a belt that ran from antarctica through sri lanka and india into madagascar this belt was involved in pan african tectono metamorphism as reflected in the 550 ma age of the last granulite forming event throughout the belt

tomography from 26 years of seismicity revealing that the spatial extent of the yellowstone crustal magma reservoir extends well beyond the yellowstone caldera the yellowstone volcanic field has experienced three of earth s most explosive volcanic eruptions in the last 2 1 ma the most recent eruption occurred 0 64 ma forming the 60 km long yellowstone caldera we have compiled earthquake data from the yellowstone seismic network from 1984 to 2011 and tomographically imaged the three dimensional p wave velocity vp structure of the yellowstone volcanic system the resulting model reveals a large low vp body interpreted to be the crustal magma reservoir that has fueled yellowstone s youthful volcanism our imaged magma body is 90 km long 5 17 km deep and 2 5 times larger than previously imaged the magma body extends 15 km ne of the caldera and correlates with the location of the largest negative gravity anomaly a 80 mgal gravity low this new seismic image provides important constraints on the dynamics of the yellowstone magma system and its potential for future volcanic eruptions and earthquakes

constraints on the upper crustal magma reservoir beneath yellowstone caldera inferred from lake seiche induced strain observations 1 seiche waves in yellowstone lake with a 78 minute period and heights 10 cm act as a load on the solid earth observed by borehole strainmeters with subnanostrain sensitivity throughout the yellowstone caldera the far field strain induced by the load of the seiche waves calculated with a homogeneous elastic model representing the upper crust is more than an order of magnitude smaller than the measured strain amplitude 30 km from the lake shore by contrast the observed far field strain amplitudes are consistent with the seiche load on a two layered viscoelastic model representing an elastic upper crust overlying a partially molten body deeper than 3 6 km with maxwell viscosity less than 1011 pa s these strain observations and models provide independent evidence for the presence of partially molten material in the upper crust consistent with seismic tomography studies that inferred 10 30 melt fraction in the upper crust

petrology and fe ti oxide reequilibration of the 1991 mount unzen mixed magma a dacitic magma 64 5 wt sio2 a mixture of phenocryst rich rhyodacite and an aphyric mafic magma was erupted during the recent 1991 1995 mount unzen eruptive cycle the experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas and the nature of the mixing process the preeruption rhyodacitic magma was at a temperature of 790 20 c according to fe ti oxide phenocryst cores and at a depth of 6 to 7 km 160 mpa according to al in hornblende geobarometry the mafic magma that mixed with the rhyodacite is found as andesitic 54 to 62 wt sio2 enclaves in the erupted magma and was essentially aphyric when intruded phase equilibria indicate that an aphyric andesite at 160 mpa is 1030 c h2o saturated and possibly as high as 1130 c 2 wt h2o the composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt sio2 using these compositions and temperatures the temperature of the unzen magma after mixing is estimated to be at least 850 to 870 c the groundmass fe ti oxide microphenocrysts and those in pargasite bearing reaction zones around biotite phenocrysts both give 890 20 c temperatures the oxide oxide contacts give temperatures of 910 20 c the 900 30 c postmixing temperatures are consistent with phase equilibria experiments which show that the magma was not above 930 c at 160 mpa our fe ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption which is a shorter time than is calculated using available diffusion data there is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma

linking rapid magma reservoir assembly and eruption trigger mechanisms at evolved yellowstone type supervolcanoes the geological record contains evidence of volcanic eruptions that were as much as two orders of magnitude larger than the most voluminous eruption experienced by modern civilizations the a d 1815 tambora indonesia eruption perhaps nowhere on earth are deposits of such supereruptions more prominent than in the snake river plain yellowstone plateau srp yp volcanic province northwest united states while magmatic activity at yellowstone is still ongoing the heise volcanic field in eastern idaho represents the youngest complete caldera cycle in the srp yp and thus is particularly instructive for current and future volcanic activity at yellowstone the heise caldera cycle culminated 4 5 ma ago in the eruption of the 1800 km3 kilgore tuff accessory zircons in the kilgore tuff display significant intercrystalline and intracrystalline oxygen isotopic heterogeneity and the vast majority are 18o depleted this suggests that zircons crystallized from isotopically distinct magma batches that were generated by remelting of subcaldera silicic rocks previously altered by low 18o meteoric hydrothermal fluids prior to eruption these magma batches were assembled and homogenized into a single voluminous reservoir u pb geochronology of isotopically diverse zircons using chemical abrasion isotope dilution thermal ionization mass spectrometry yielded indistinguishable crystallization ages with a weighted mean 206pb 238u date of 4 4876 0 0023 ma mswd 1 5 n 24 these zircon crystallization ages are also indistinguishable from the sanidine 40ar 39ar dates and thus zircons crystallized close to eruption this requires that shallow crustal melting assembly of isolated batches into a supervolcanic magma reservoir homogenization and eruption occurred extremely rapidly within the resolution of our geochronology 103 104 yr the crystal scale image of the reservoir configuration with several isolated magma batches is very similar to the reservoir configurations inferred from seismic data at active supervolcanoes the connection of magma batches vertically distributed over several kilometers in the upper crust would cause a substantial increase of buoyancy overpressure providing an eruption trigger mechanism that is the direct consequence of the reservoir assembly process

origin and evolution of silicic magmatism at yellowstone based on ion microprobe analysis of isotopically zoned zircons the origin of large volume yellowstone ignimbrites and smallervolume intra caldera lavas requires shallow remelting of enormous volumes of variably o depleted volcanic and sub volcanic rocks altered by hydrothermal activity zircons provide probes of these processes as they preserve older ages and inherited do values this study presents a high resolution oxygen isotope examination of volcanism at yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size we report 357 analyses of cores and rims of zircons and isotope profiles of 142 single zircons in 11 units that represent majoryellowstone ignimbrites and post caldera lavas many zircons from these samples were previously dated in the same spots by sensitive high resolution ion microprobe shrimp and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination we additionally report oxygen isotope analyses of quartz crystals in three units the results of this work provide the following new observations 1 most zircons from post caldera low do lavas are zoned with higher do values and highly variable u pb ages in the cores that suggest inheritance from pre caldera rocks exposed on the surface 2 many of the higher do zircon cores in these lavas have u pb zircon crystallization ages that postdate caldera formation but pre date the eruption age by 10 20 kyr and represent inheritance of unexposed post caldera sub volcanic units that have do similar to the lava creek tuff 3 young and voluminous 0 25 0 1ma intra caldera lavas which represent the latest volcanic activity atyellowstone contain zircons with both high do and lowdo cores surrounded by an intermediate do rim this implies inheritance of a variety of rocks from high do pre caldera and low do post caldera units followed by residence in a common intermediate do melt prior to eruption 4 major ignimbrites of huckleberry ridge and to a lesser extent the lava creek and mesa falls tuffs contain zoned zircons with lower do zircon cores suggesting that melting and zircon inheritance from the lowdo hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse 5 the do zoning in the majority of zircon core rim interfaces is step like rather than smoothly inflected suggesting that processes of solution reprecipitation were more important than intracrystalline oxygen diffusion concave downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals this study suggests that silicic magmatism atyellowstone proceeded via rapid shallow level remelting of earlier erupted and hydrothermally alteredyellowstone source rocks and that pulses of basaltic magma provided the heat for melting each postcaldera yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and do values the commonly held model of a single large volume super solidus mushy state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data rather the source rocks for theyellowstone volcanism were cooled below the solidus hydrothermally altered by heated meteoric waters that caused low do values and then remelted in distinct pockets by intrusion of basic magmas each packet of new melt inherited zircons that retained older age and do values this interpretation may have significance for interpreting seismic data for crustal low velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side new basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas we also suggest that the lava creek tuff

decoupled crystallization and eruption histories of the rhyolite magmatic system at tarawera volcano revealed by zircon ages and growth rates we obtained u th disequilibrium age data on zircons from each of the four rhyolite eruptions that built tarawera volcano in the last 22 ka within the okataina volcanic center ovc caldera new zealand secondary ion mass spectrometry analyses on unpolished euhedral crystal faces that lack resorption features show that crystal growth variously terminated from near eruption age to 100 ka prior to eruption age depth profiling of crystals reveals long periods of continuous 34 ka and discontinuous growth 90 ka growth hiatuses of up to 40 ka duration occur but do not all relate to obvious resorption surfaces age differences up to similar magnitude are found on opposing faces of some crystals suggesting episodes of partial exposure to melts these features are best explained by periodic complete or partial sub solidus storage and or inclusion in larger crystal phases followed by rapid liberation prior to eruption this is supported by high abundances of u and th 500 2 000 ppm in some zircons consistent with periods of high crystallinity 70 in the magmatic system based on crystal melt partitioning contemporaneous but contrasting rim ward trends of these elements within crystals even in the same lava hand sample require synchronous growth in separate melt bodies and little connectivity within the system but also significant crystal transport and mixing prior to eruption many crystals record continuity of growth through the preceding 60 ka ovc caldera collapse and subsequent eruptions from tarawera this demonstrates a decoupling between eruption triggers such as shallow crustal extension and mafic intrusion and the crystallization state of the ovc silicic magmatic system the data highlights the need to distinguish between the time for accumulation of eruptible magma and the long term magma residence time based on the age of crystals with high closure temperatures when assessing the potential for catastrophic eruptions

crystal scale anatomy of a dying supervolcano an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the yellowstone caldera a voluminous 600 km3 and long lived 520 75 ka phase of rhyolitic eruptions followed collapse of the yellowstone caldera 640 ka whether these eruptions represent a dying cycle or the growth of a new magma chamber remains an important question we use new u th zircon ages and 18o values determined by ion microprobe and sanidine pb isotope ratios determined by laser ablation to investigate the genesis of voluminous post caldera rhyolites the oldest post caldera rhyolites erupted between 520 and 470 ka exhibit extreme age and oxygen isotopic heterogeneity requiring derivation from individual parcels of low 18o melts we find a progressive increase in zircon homogeneity for rhyolite eruptions from 260 to 75 ka with homogeneous low 18o zircon values of 2 7 2 8 that are in equilibrium with low 18o host melts for the majority of the youngest eruptions new sanidine pb isotope data define separate arrays for post caldera rhyolites and the caldera forming tuffs that preceded them indicating that they were not sourced from a mushy lava creek tuff batholith that remained after caldera collapse rather our new age and isotopic data indicate that the post caldera rhyolites were generated by remelting of a variety of intracaldera source rocks consisting of pre lava creek tuff volcanic and plutonic rocks and earlier erupted post lava creek tuff rhyolites batch assembly of low 18o melts starting at 260 ka resulted in progressive homogenization followed by differentiation and cooling up until the last rhyolite eruption 75 ka a trend that we interpret to be characteristic of a dying magma reservoir beneath the yellowstone caldera

magma mixing and the generation of isotopically juvenile silicic magma at yellowstone caldera inferred from coupling 238 u 230 th ages with trace elements and hf and o isotopes in zircon and pb isotopes in sanidine the nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time we examine compositional heterogeneity in the youngest 170 to 70 ka post caldera volcanism at yellowstone caldera the central plateau member cpm rhyolites as a case study we compare 238u 230th age trace element and hf isotopic data from zircons and major element ba and pb isotopic data from sanidines hosted in two cpm rhyolites hayden valley and solfatara plateau flows and one extracaldera rhyolite gibbon river flow all of which erupted near the caldera margin ca 100 ka the hayden valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the cpm reservoir the gibbon river flow hosts one zircon population that is compositionally distinct from hayden valley flow zircons the solfatara plateau flow contains multiple sanidine populations and all three zircon populations found in the hayden valley and gibbon river flows demonstrating that the solfatara plateau flow formed by mixing extracaldera magma with the margin of the cpm reservoir this process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs more generally hf isotopic data from the cpm zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post caldera magmatic system and demonstrate that the sources contributing magma to the cpm reservoir were heterogeneous in 176hf 177hf at ca 100 ka thus the limited compositional variability of cpm glasses reflects homogenization occurring within the cpm reservoir not a homogeneous source

arrested charnockite formation at kottavattam southern india at kottavattam southern kerala india late proterozoic homogeneous leptynitic garnet biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse grained massive charnockite sensu stricto along a set of conjugate fractures transecting the gneissic foliation charnockitization post dates the polyphase deformation regional high grade metamorphism and anatexis and evidently occurred at a late stage of the pan african tectonothermal history geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similar p lith t conditions 650 700 degrees c 5 6kbar in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases x co2 0 7 0 6 x h2o 0 2 0 3 x n2 0 1 logf o2 l7 5 mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow outward migrating transition zones adjacent to the gneiss followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone compared to the host gneiss the charnockite shows higher concentrations of k na sr ba and zn and lower concentrations of mg fe ti v y zr and the hree with a complementary pattern in the narrow transition zones of biotite breakdown the p lith t x h2o data and chemical zonation patterns indicate charnockitization through subsolidus dehydration reaction in an open system subsequent residence of the carbonic fluids in the charnockite resulted in low grade alteration causing modification of the syn charnockitic elemental distribution patterns and the properties of entrapped fluids we favour an internally controlled process of arrested charnockitization in which during near isothermal uplift the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from fluid absent in the gneiss to fluid present in the fracture zones and to the development of an initial fluid pressure gradient triggering the dehydration reaction

pan african granitic magmatism in the kerala khondalite belt southern india abstract granitic rocks in the kerala khondalite belt kkb of southern india can be distinguished according to their chemical composition and their mode of formation porphyritic granites now present as coarse grained augen gneisses represent an early stage of pan african granitic magmatism which most likely occurred at amphibolite facies conditions u pb and epma monazite dating suggest a minimum intrusion age of 590 ma for the granitic precursors of the augen gneisses garnet and biotite bearing leucogranites are abundant throughout the whole kkb and were formed from high t dehydration melting of the various kkb gneisses at granulite facies conditions subsequent to the main stage of pan african ductile deformation fine grained granites some of them with distinct alkali affinities are abundant in the achankovil unit and formed pre to post tectonically available geochronological data point to an emplacement c 560 ma ago or later corresponding petrological and field data suggest that the leucogranites were formed and emplaced during the peak stage of pan african metamorphism whereas the achankovil granites partly may reflect extensional tectonics and lithospheric thinning

revised ages for tuffs of the yellowstone plateau volcanic field assignment of the huckleberry ridge tuff to a new geomagnetic polarity event 40 ar 39 ar ages were determined on the three major ash flow tuffs of the yellowstone plateau volcanic field in the region of yellowstone national park in order to improve the precision of previously determined ages total fusion and incremental heating ages of sanidine yielded the following mean ages huckleberry ridge tuff 2 059 0 004 ma mesa falls tuff 1 285 0 004 ma and lava creek tuff 0 639 0 002 ma the huckleberry ridge tuff has a transitional magnetic direction and has previously been related to the reunion normal polarity subchron dating of the reunion event has been reviewed and its ages have been normalized to a common value for mineral standards the age of the huckleberry ridge tuff is significantly younger than lava flows of the reunion event on re union island supporting other evidence for a normal polarity event younger than the reunion event