prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

chemistry of springs across the mariana forearc shows progressive devolatilization of the subducting plate abstract cold springs upwelling through large serpentinite mud volcanoes in the outer half of the mariana forearc provide a unique window into processes of devolatilization of the subducting pacific plate we have sampled upwelling pore waters with lower chlorinity than seawater from six sites on five serpentinite mud volcanoes by conventional gravity and piston coring by push coring from the rov jason by drilling on odp legs 125 and 195 and by manned submersible the sites range from 13 47 n to 19 33 n and 52 to 90 km from the mariana trench axis corresponding to approximate depths to the top of the downgoing plate of 16 to 29 km the composition of the springs varies systematically over this distance nearer the trench the upwelling waters have much higher ca and sr than seawater and much lower carbonate alkalinity sulfate na cl k rb and b farther from the trench the waters show the opposite trends relative to seawater chlorinity is consistently lower than in seawater and shows large variations that are not systematic with distance from the trench cs is consistently higher than in seawater and increases with distance from the trench all of the waters have high ph and are heavily depleted in mg si li f and 87sr 86sr relative to seawater they tend to be enriched in o18 o16 except for odp drilling none of the cores was long enough to produce an asymptotic compositional trend with depth we have inferred the end member compositions of the upwelling waters by extrapolation against mg at two sites we were able to compare data from gravity cores with data from drill cores or push cores collected at springs to estimate the effects of reactions that occur at shallow depth below the seafloor on mixing of the upwelling waters with seawater these effects are different for sites high in dissolved ca nearer the trench vs those high in alkalinity farther from the trench common to both are large losses from solution of 1 ca as caco3 and in exchange for na 2 mg in exchange for na or ca and as brucite 3 sulfate probably reduced by microbes or possibly precipitated as gypsum 4 sr ba si and f na is consistently leached from the solids into solution whereas k and o18 o16 are relatively unreactive we infer that the upwelling waters are uniformly saturated with caco3 and that the excess h2o and the trends in ca sr alkalinity and sulfate with distance from the trench result from introduction of h2o and dissolved carbonate and sulfate from an external source the sediment and altered basalt at the top of the subducting plate the concurrent trends in na cl b cs and especially k and rb indicate that these species originate from the top of the subducting plate in response to increasing temperature these systematic variations across the outer forearc imply that the solutions ascend more or less vertically from the source region and do not travel long distances laterally along the decollement before ascending based on leaching of k the 150 c isotherm is crossed approximately beneath big blue seamount at a depth of 22 km below the seafloor 70 km behind the trench by this point it appears that carbonate dissolution has joined dehydration as a significant process at the top of the subducting plate

is the valles caldera entering a new cycle of activity the valles caldera formed during two major rhyolitic ignimbrite eruptive episodes the bandelier tuff at 1 61 and 1 22 ma after some 12 m y of activity in the jemez mountains volcanic field new mexico several subsequent eruptions between 1 22 and 0 52 ma produced dominantly high silica rhyolite lava domes and tephras within the caldera these were followed by a dormancy of 0 46 m y prior to the most recent intracaldera activity the longest hiatus since the inception of the bandelier magma system at 1 8 ma the youngest volcanic activity at 60 ka produced the sw moat rhyolites a series of lavas and tuffs that display abundant petrologic evidence of being newly generated melts petrographic textures conform closely to published predictions for silicic magmas generated by intrusion of basaltic magma into continental crust the valles caldera may currently be the site of renewed silicic magma generation induced by intrusion of mafic magma at depth recent seismic investigations revealed the presence of a large low velocity anomaly in the lower crust beneath the caldera the generally aseismic character of the caldera despite abundant regional seismicity may be attributed to a heated crustal column the local effect of 13 m y of magmatism and emplacement of mid crustal plutons seismic signals of magma movement in the deep to mid crust may therefore be masked and clear seismic indications of intrusion may only be generated within a few kilometres of the surface we therefore encourage the establishment of a local dedicated volcanic monitoring system

new evidence for the age of the youngest eruptions in the valles caldera new mexico new geochronologic data provide evidence for an age of about 50 to 60 ka for the youngest volcanic eruptions within the valles caldera new mexico an age that is significantly younger than most previous age determinations thermoluminescence age estimates for buried soils beneath the el cajete pumice a key stratigraphic marker in the region range from 48 to 61 ka and sup 14 c analyses of burnt logs within volcanic surge beds near the el cajete vent yield similar ages of 50 to 58 ka these data conflict with fissontrack sup 40 ar sup 39 ar and k ar ages of 130 to 200 ka but are supported by recent analyses by electron spin resonance the results of this study reinforce the need to apply a variety of dating methods when evaluating the age of young volcanic events and support the hypothesis that the el cajete eruptions were part of a new cycle of volcanic activity in the valles caldera after an exceptionally long period of quiescence of nearly 460 ka the new age constraints also suggest a previously unrecognized link between cycles of volcanism and pulses of hydrothermal activity in the caldera such that hydrothermal outflow appears to decrease significantly following completion ofmore eruptive cycles 24 refs 3 figs 2 tabs less

reconstruction of the most recent volcanic eruptions from the valles caldera new mexico abstract products of the latest eruptions from the valles caldera new mexico consist of the el cajete pyroclastic beds and battleship rock ignimbrite a sequence of pyroclastic fall and density current deposits erupted at 55 ka capped by the later banco bonito flow erupted at 40 ka and collectively named the east fork member of the valles rhyolite the stratigraphy of the east fork member has been the subject of conflicting interpretations in the past a long running investigation of short lived exposures over a period of many years enables us to present a more complete event stratigraphy for these eruptions than has hitherto been possible the volume of rhyolitic magma erupted during the 55 ka event may have been more than 10 km3 and for the 40 ka event can be estimated with rather more confidence at 4 km3 during the earlier event plinian eruptions dispersed fallout pumice over much of the valles caldera the southern jemez mountains and the rio grande rift we infer a fallout thickness of several decimeters at the site of the city of santa fe and significant ash fall in eastern new mexico in contrast pyroclastic density currents were channeled within the caldera moat and southwestward into the head of canon de san diego the principal drainage from the caldera simultaneous or rapidly alternating pyroclastic fallout and density current activity characterized the 55 ka event with density currents becoming more frequent as the eruption progressed through two distinct stages separated by a brief hiatus one early pyroclastic surge razed a forest in the southern caldera moat in a similar manner to the initial blast of the may 18 1980 eruption of mt st helens ignimbrite outflow from the caldera through the drainage notch may have been restricted in runout distance due to steep rugged topography in this vicinity promoting mixing between flows and air and the formation of phoenix clouds lavas erupted during both the 55 and 40 ka events were largely confined to the caldera moat any future rhyolitic eruptions of similar magnitude in the southern or western parts of the valles caldera will likely affect similar areas

the eruptive and magmatic history of the youngest pulse of volcanism at the valles caldera implications for successfully dating late quaternary eruptions abstract new 40ar 39ar and u th ages provide insight into the youngest eruptions at the valles caldera and also reveal previously unknown pulses of magmatism the youngest eruptive units collectively termed the east fork member of the valles rhyolite include the el cajete pyroclastic beds and co erupted battleship rock ignimbrite and the disconformably overlying banco bonito lava flow previous attempts to date these units using a variety of techniques yielded ages ranging from 1 ma new 40ar 39ar ages were generated using the high sensitivity multi collector argus vi mass spectrometer which provides more than an order of magnitude increase in precision compared to most single detector mass spectrometers 40ar 39ar dating of single crystals yields a range of ages of which the youngest populations are interpreted to represent the eruption age sanidine ages indicate that the el cajete pyroclastic beds and battleship rock ignimbrite erupted at 74 4 1 3 ka whereas the banco bonito lava erupted at 68 3 1 5 ka populations of older crystals represent variably degassed xenocrysts explaining why previous 40ar 39ar bulk step heating analyses yielded spuriously old and irreproducible results u th dating of unpolished zircon surfaces also yield multiple age populations which range from the eruption age to 350 ka indicating protracted magmatism during the 453 ka long eruptive hiatus prior to the eruption of the east fork member 40ar 39ar and u th ages indicate that the east fork member represents a short 6 1 2 8 ka eruptive cycle from a longer lived magmatic system beneath the southern caldera equally short repose periods similar to the interval between the el cajete battleship rock and banco bonito eruptions are possible during future volcanism at the valles caldera results demonstrate that detailed geochronology using single crystal and in situ techniques is necessary for understanding the eruptive history and magmatic evolution at some young volcanic systems

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

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

migration of volcanism in the san francisco volcanic field arizona the remanent magnetization of volcanic rocks has been determined at 650 sites in the san francisco volcanic field in the southern part of the colorado plateau the polarity of remanent magnetization combined with k ar age determinations spatial and petrographic associations stratigraphic relations and state of preservation of the cinder cones provides a basis for assignment to known magnetic polarity epochs of 610 mafic vents and 100 intermediate to silicic flows flow sequences and vents the age assignments for basaltic rocks include 243 brunhes 0 73 ma vents 220 matuyama 0 73 to 2 48 ma vents and 147 pre matuyama 2 48 to about 5 0 ma vents basaltic volcanism migrated northeastward before matuyama time at a rate of 1 2 cm yr and eastward s87 5 e over the past 2 5 m y at a rate of 2 9 0 3 cm yr concomitant acceleration in total magma production from 75 to 1 400 10 6 km3 yr and frequency of basaltic eruptions from 1 per 17 000 yr to 1 per 3 000 yr occurred between 5 and 0 25 ma for the past 0 25 m y magma production 180 10 6 km3 yr and perhaps eruption frequency have decreased this evolutionary sequence coupled with the lead and strontium isotopic composition of the rocks can be explained by magmatism caused by shear heating at the base of the lithosphere we propose that this eastward drift of volcanic activity represents absolute westward motion of the north american plate our model is in agreement with a model in which the african plate is fixed to the deep mantle

the 40ar 39ar age constraints on the duration of resurgence at the valles caldera new mexico 1 the 40ar 39ar ages for rocks from the valles caldera new mexico imply that resurgent uplift of the caldera floor occurred within 27 27 ka of caldera collapse the structural resurgent dome of the valles caldera was uplifted approximately 1000 m above the surrounding caldera floor during resurgence the upper bandelier tuff which yields an age of 1 256 0 010 ma 2 ages relative to fish canyon tuff sanidine at 28 02 ma is the lower constraint on the timing of resurgence the oldest postcollapse ring fracture dome cerro del medio is the upper time constraint and yields an age of 1 229 0 017 ma for one of its oldest flow lobes therefore resurgent uplift probably occurred within 54 ka at a minimum rate of 1 9 cm yr sanidine phenocrysts from rhyolites of the postcollapse intracaldera deer canyon and redondo creek members yield 40ar 39ar single crystal laser fusion ages ranging from 1 229 0 013 ma to 1 283 0 017 ma deer canyon member n 8 and 1 208 0 017 ma to 1 239 0 017 ma redondo creek member n 4 with one exception from each unit these ages are statistically indistinguishable from the upper bandelier tuff indicating that eruption of these postcollapse rhyolites probably commenced shortly after caldera formation melt inclusion hosted excess argon was not found to have a measurable effect on the age of sanidine from the upper bandelier tuff or the deer canyon member

thermodynamics of the amphiboles ca mg fe2 quadrilateral a thermodynamic model has been developed and calibrated for monoclinic and orthorhombic amphiboles compositionally contained within the ca mg fe 2 amphibole quadrilateral the model incorporates the energetic consequences of cation ordering of fe 2 and mg over nonequivalent sites in the crystal structure and accounts for the temperature pressure and compositional dependence of the orthorhombic monoclinic phase transition calibration is based on previously published work on the thermodynamic properties of fe 2 mg amphiboles and tremolite and experimental cation ordering data along with solvus width and tieline orientations of natural coexisting amphiboles in the quadrilateral among derived parameters is the enthalpy of formation of end member ferroactinolite 10534 966 kj mol a calculated femg 1 isopotential solvus at mid composition in the quadrangle agrees well with a revised calibration of experimental data from cameron 1975 the solvus is not strongly asymmetric and narrows with increasing fe mg ratio and temperature phase relations along the mg and fe sides of the quadrilateral are compared and contrasted cummingtonite anthophyllite phase relations are shown in the t x f e plane with and without coexisting calcic amphibole the breakdown reactions of quadrilateral amphiboles to assemblages of two pyroxenes olivine quartz and h 2 o are depicted as functions of temperature pressure composition h 2 o activity and oxygen fugacity some highlights of their relevance to metamorphic and igneous systems are discussed

1 spatial and temporal evolution of the optical thickness of the pinatubo aerosol cloud in the northern hemisphere from a network of ship borne and stationary lidars the vertical profiles of the extinction coefficient and the total optical thickness of the pinatubo aerosol layer obtained from a network of 5 rayleigh mie lidars are presented here three ship borne lidars professor zubov professor vize henri poincare and two fixed lidar stations ohp and cel are operated respectively by the roscomhydromet of russia and of the service d aeronomie du cnrs of france the measurements presented are in the altitude range 15 35 km they were obtained between july 1991 april 1992 and cover 8 s 60 n latitude and 80 w 6 e longitude this represents extensive coverage of the western sector of the northern hemisphere which is partly coincident with uars satellite coverage optical depths of up to 0 2 were observed and maximum extinction coefficient values of 0 08 km 1 were obtained at 24 km and 18 n latitude

proto polymorphs of jimthompsonite and chesterite in contact metamorphosed serpentinites from japan proto polymorphs of jimthompsonite and chesterite occur in metamorphosed serpentinites from two japanese ultramafic complexes the lattice constants of the proto triple chain silicate measured by x ray diffraction are a 0 93605 208 b 2 72560 588 and c 0 53160 89 nm whereas those of the mixed double and triple chain silicate are a 0 94202 78 b 4 54402 392 c 0 53440 45 nm and 90 026 18 the lattice constants and systematic extinctions revealed by selected area diffraction patterns are consistent with proto triple chain silicate pbcn and mixed double and triple chain silicate a 2 m am but not with the ortho and clino polymorphs high resolution transmission electron microscopy 016 images of the triple chain silicate and 0 1 15 images of the mixed chain silicate indicate they have a x configuration proto forms of wide chain pyriboles might be geologically widespread

excess argon in melt inclusions in zero age anorthoclase feldspar from mt erebus antarctica as revealed by the method abstract historically erupted 1984 anorthoclase phenocrysts from mt erebus yield k ar and 40 ar 40 ar apparent ages as old as 700 ka indicating the presence of excess argon 40 ar 39 ar furnace step heating results from anorthoclase reveal a positive correlation between the cl k ratio and apparent age because chlorine up to 1700 ppm is present in melt inclusions but not in the anorthoclase crystal lattice this correlation suggests that the excess argon is associated with melt inclusions trapped within the anorthoclase during rapid crystal growth confirmation of the source of excess argon comes from step heating experiments on multiple anorthoclase aliquots separated from two phenocrysts and one glass aliquot prepared from the matrix of a volcanic bomb the anorthoclase phenocrysts were crushed and hf etched to yield aliquots containing 30 10 and 1 melt inclusions the step heated anorthoclase with 30 and 10 melt inclusions yielded the highest cl k ratios and apparent integrated ages of 640 30 ka and 179 16 ka respectively the anorthoclase with i melt inclusions yielded significantly lower cl k ratios and apparent integrated ages of 48 8 ka and 50 30 ka the step heated volcanic glass yielded the least variable cl k ratios and a total gas age of 101 16 ka argon released from the anorthoclase and the trapped melt inclusions can be distinguished by differences in their degassing behavior allowing geologically more reasonable ages to be obtained melt inclusions exposed on the exterior of anorthoclase grains principally degas during furnace extraction at temperatures less than 1200 c inclusions entirely within anorthoclase grains principally degas at temperatures greater than 1200 c when incongruent melting of the anorthoclase allows melt inclusion hosted excess argon to escape anorthoclase aliquots prepared with less than 1 inclusions can be fitted with a plateau for heating steps below 1200 c to yield ages as young as 8 2 ka whereas steps above 1200 c yield ages in excess of 100 ka however anorthoclase aliquots containing 10 30 melt inclusions yield ages in excess of 200 ka for heating steps below 1200 c minimizing the effects of the excess argon from melt inclusions relies on sample preparation and step heating fine crushing and treatment with hydrofluoric acid removes many of the larger melt inclusions small melt inclusions which remain within the anorthoclase degas primarily at temperatures above 1200 c temperatures below 1200 c yield the most accurate ages attempts at post analytically correcting for the chlorine correlated excess argon are hindered by the variations in 40 ar e cl within and between samples elevated 40 ar e cl ratios in bubbles within the melt inclusions as deduced from in vacuo crushing experiments are the most likely cause for some or all of a sample s total 40 ar e cl variation in addition relative solubilities of argon and chlorine within phonolitic melts may be partly responsible for variations in 40 ar e cl

crust and upper mantle p wave velocity structure beneath valles caldera new mexico results from the jemez teleseismic tomography experiment new results are presented from the teleseismic component of the jemez tomography experiment conducted across valles caldera in northern new mexico we invert 4872 relative ital p wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km the three principle features of our model for valles caldera are 1 near surface low velocities of minus 17 percent beneath the toledo embayment and the valle grande 2 midcrustal low velocities of minus 23 percent in an ellipsoidal volume underneath the northwest quadrant of the caldera and 3 a broad zone of low velocities minus 15 percent in the lower crust or upper mantle crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast near surface low velocities are interpreted as thick deposits of bandelier tuff and postcaldera volcaniclastic rocks lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast beneath the valle grande we interpret the midcrustal low velocity zone to contain a minimum melt fraction of 10 percent while we cannot rule out the possibility that this zone is the remnant 1 2 ma bandelier magma chamber the eruption history and geochemistry of the volcanic rocks erupted in valles calderamore following the bandelier tuff make it more likely that magma results from a new pulse of intrusion indicating that melt flux into the upper crust beneath valles caldera continues the low velocity zone near the crust mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle in either case this low velocity anomaly indicates that underplating by mantle derived melts has occurred copyright 1998 american geophysical union less

ultra high precision 40ar 39ar ages for fish canyon tuff and alder creek rhyolite sanidine new dating standards required abstract the 40 ar 39 ar dating technique is a high precision 40 ar 39 ar standards include sanidine from the ca 28 ma fish canyon tuff fct and the ca 1 2 ma alder creek rhyolite acr despite common usage the ages of fct and acr sanidine have proven contentious with reported values varying by 2 well outside the 0 1 aspiration of earthtime www earth time org the current study presents ultra high precision multi collector 40 ar 39 ar results for fct and acr sanidine using a new generation argusvi mass spectrometer these analyses demonstrate the significantly higher by a factor of 10 level of precision achievable using the argusvi system compared to most previous studies significantly 40 ar 39 ar step heating analyses resolve distinct age gradients for both fct and acr sanidine possible explanations for these gradients include combinations of recoil loss of 39 ar instrumental bias isotopic fractionation during step heating thermally induced argon loss and or extraneous argon contamination minor recoil loss of 39 ar and partial retention of inherited argon during extended magma residence are deemed the most plausible causes of the age discordance given this assumption we calculate revised maximum eruption ages of 28 01 0 04 ma 2 for fct and 1 178 0 002 ma 2 for acr relative to the astronomically calibrated age of a1 tephra sanidine crete rivera et al 2011 earth planet sci lett 311 420 426 nonetheless the argusvi data demonstrate that fct and acr sanidine are non ideal as high precision 40 ar 39 ar standards with different heating protocols likely to induce inter laboratory bias the current findings account for at least some of the scatter in inter calibration ages reported previously for various 40 ar 39 ar standards in broader terms the results mandate a re evaluation of the astronomically calibrated ages for these and other sanidine samples while the new generation of multi collector mass spectrometers provides the means to further evaluate 40 ar 39 ar ages used to define new decay constants and standard ages

the el cajete series valles caldera new mexico three rhyolite eruptive units totaling 3 km3 dense rock equivalent volume are the youngest products from the valles caldera new mexico these pyroclastic and effusive units herein called the el cajete series were erupted over an appreciable time period with plinian and ignimbrite forming activity preceding an effusive phase by a lengthy but indeterminable interval new u th disequilibrium and published fission track dates support an age for the event between 170 and 130 ka the explosive phases formed the el cajete plinian deposit about 1 3 km3 minor dry pyroclastic surges and the battleship rock tuff about 1 0 km3 a valley confined welded ignimbrite the plinian eruption column is estimated to have been 28 km high during deposition of the most widely dispersed pumice fall unit slow effusion of the banco bonito obsidian lava flow 1 0 km3 onto a dissected surface cut into the el cajete and battleship rock pyroclastic deposits accompanied by minor explosive activity terminated the event twenty four bulk rock samples from all three eruptive stages exhibit little significant compositional variation and the three units are petrographically identical apart from differences arising from contrasting eruption styles very few phenocrysts appear to have been in equilibrium with their enclosing high silica rhyolitic liquid plagioclase grains are typically strongly resorbed while biotite and hornblende frequently occur in aggregate grains in which textures characteristic of plutonic rocks can be discerned these features result from partial melting of preexisting crustal igneous rock probably of dioritic to granodioritic composition and subsequent eruption of the resulting liquid plus restite crystals rapid generation and eruption of rhyolitic magma during this most recent phase of activity in the jemez mountains may imply that the valles magma system is presently in a state where small magma bodies are transient phenomena

teleseismic p wave image of crust and upper mantle structure beneath the valles caldera new mexico initial results from the 1993 jtex passive array teleseismic p wave relative arrival time data collected from a temporary array during the 1993 jemez tomography experiment jtex have been inverted to image velocity anomalies beneath the valles caldera in northern new mexico instruments were deployed in two 30 km long profiles one of 8 and one of 9 stations these profiles crossed the caldera trending at azimuths of n46 w and n60 e respectively two dimensional teleseismic relative arrival time inversion of the 1993 data set supplemented with data from an overlapping 1987 profile confirms the existence of a mid crustal low velocity region 30 beneath the valles caldera in the depth range of 8 to 13 km below sea level with about a 6 km horizontal extent this feature is interpreted to be the seismic expression of the remnant magma chamber a shallow low velocity anomaly beneath san antonio mountain coincides with the region of highest thermal gradient values in the caldera a lower crust upper mantle low velocity anomaly is imaged but is not as well constrained due to the limited length of the profile we tentatively correlate this anomaly with the thermal effects of basaltic magmas ponded at the crust mantle boundary

formation conditions for triple chain silicates since the first report of triple chain silicates in 1977 there has been considerable uncertainty about their formation in nature primarily concerning whether they are stable minerals or an intermediate step in the formation of other chain or sheet silicate minerals the lack of thermochemical data and relevant experimental studies for any triple chain silicate has impeded establishment of their thermodynamic stability relative to other minerals here we combine recently reported thermochemical data for a sodium analog of the triple chain silicate clinojimthompsonite na cjt with existing data for various common minerals i e enstatite talc anthophyllite and chrysotile we also present new experimental data to show that na cjt should form stably at typical metamorphic conditions of 0 2 gpa and 450 c ion activity vs ph diagrams calculated at 100 300 c and 0 1 gpa show that na cjt could form by reaction between primary igneous minerals enstatite forsterite in oceanic harzburgite and seawater at temperatures up to 300 c however formation of na cjt from harzburgite that has been altered to talc and chrysotile is not expected to occur except at the unusually high ph conditions observed for forearc serpentine mounds estimates of the thermochemical properties of the na free end member clinojimthompsonite mg 10 si 12 o 32 oh 4 suggest that it also should form from the primary mineralogy of harzburgite at temperatures and si concentrations typically observed for various oceanic hydrothermal and geothermal systems but probably not from ultramafic rock already hydrated to serpentine

high resolution tem study of jimthompsonite chesterite and chain width disorder in archean ultramafic rocks from isua west greenland jimthompsonite and chesterite occur as intergrowths within anthophyllite up to a few millimeters across in talc magnesite schist from the 3800 ma isua supracrustal belt west greenland pyribole lamellae span the entire intergrowth width and have their own crystal faces the schist is strongly foliated and pyribole intergrowths are aligned with this foliation indicating broad synchronicity among metamorphic deformation recrystallization and pyribole formation chain width errors polytypic disorder zipper terminations displacive planar faults replacement products by sheet silicates polygon shaped cavities cavity filling materials and low angle grain boundaries were revealed by hrtem based on cross cutting relationships the order of the formation of these defects was determined pyribole intergrowths formed prior to displacive planar faults and cavity filling materials fine sheet silicates replaced pyriboles or filled cracks after the pyribole intergrowth formation no obvious reaction or replacement products by talc or chlorite were confirmed at the interface between pyriboles and optically recognizable large sheet silicates suggesting that fine sheet silicates replacing pyriboles formed later than talc and chlorite polysomatic and polytypic lamellae may have grown simultaneously because the lamellae cross cut each other and one does not affect the formation of the other the possible reaction fronts that separate jimthompsonite or chesterite from anthophyllite were not found in 001 hrtem images of our samples instead narrow jimthompsonite and chesterite lamellae that break termination rules were commonly found these textural features can be better understood as a simultaneous growth of jimthompsonite chesterite and anthophyllite and not as a retrograde reaction product of anthophyllite