Time scales of large volume silicic magma systems: Sr isotopic systematics of phenocrysts and glass from the Bishop Tuff,Long Valley,California

The initial Sr isotopic compositions of glass and mineral separates from the 0.74 Ma Bishop Tuff ashflow in eastern California were determined to investigate the time scales of magmatic processes in a large silicic system. It was found that there is substantial isotopic heterogeneity, both between eruptive units and between glass and phenocryst phases of individual units. The frist-erupted, lower temperature units generally have higher initial ⁸⁷Sr/⁸⁶Sr than later crupted, higher temperature units. Within each unit, feldspar phenocrysts have the lowest ⁸⁷Sr/⁸⁶Sr, associated glass has higher ⁸⁷Sr/⁸⁶Sr, and biotite phenocrysts have the highest ⁸⁷Sr/⁸⁶Sr. These isotopic differences were produced by processes in the magma chamber and not by post-eruptive alteration. Two samples were similar Fe–Ti oxide temperatures but from widely separated localities have nearly identifical Sr isotopic characteristics, indicating the existence of compositionally uniform layers of substantial volume within the chamber. Trace element data indicate that the feldspars crystallized from a liquid represented by the associated glass, and that the feldspar-glass pairs are not accidental. The rhyolitic liquids of the Bishoptuff magma chamber apparently experienced increasing ⁸⁷Sr/⁸⁶Sr at a rate too fast for feldspar phenocrysts to remain in isotopic equilibrium. The increasing ⁸⁷Sr/⁸⁶Sr is caused primarily by radioactive decay of ⁸⁷Rb in the high-Rb/Sr liquids and not primarily by assimilation of radiogenic wall-rock material. A self-consistent model can be constructed to account for all of the isotopic data except for those on biotite phenocrysts. The time scale for evolution of the system is bounded on the high side at about 500 ky by observations made on precaldera lavas, and on the low side at approximately 300 ky by the time necessary to establish homogeneous layers in an actively differentiating chamber. The deduced time scale is consistent with model Rb–Sr ages, which date the differentiation of low temperature liquids from higher temperature liquids, and is compatible with the observed isotopic disequilibrium between feldspars and glass because of the low diffusivity of Sr in fieldspars (<10^-16 cm²/s). The prolonged (about 500 ky) evolution of the Bishop Tuff system was facilitated by a large influx of basaltic material (about 10^-2 km³/y) to the base of the system, which compensated for diffusive heat loss from the top and allowed large volumes of magma to maintain low crystal contents for >3x10⁵ years. The silicic-magma production rate within the Bishop Tuff magma chamber is estimated to be 10^-3km³/y. The growth rate of alkali feldspar is estimated to be about 10^-14 cm/s based on the Sr isotopic difference between sanidine and glass of the lower Bishop Tuff. The biotite population is inferred to be partially (>50 ppm) xenocrystic, the xenocrysts being introduced to the chamber less than one year prior to eruption.     

Insights into shallow magmatic processes in large silicic magma bodies: the trace element record in the Fish Canyon magma body,Colorado

Highly evolved rhyolite glass plus near-solidus mineral assemblages in voluminous, dacitic, crystal-rich ignimbrites provide an opportunity to evaluate the late magmatic evolution of granodiorite batholiths. This study reports laser-ablation ICP-MS analyses of trace element concentrations in feldspars, hornblende, biotite, titanite, zircon, magnetite, and interstitial glass of the crystal-rich Fish Canyon Tuff. The high-silica rhyolite glass is characterized by relatively high concentrations of feldspar-compatible elements (e.g., 100 ppm Sr and 500 ppm Ba) and low concentrations of Y (<7 ppm) and HREE (~1 ppm Yb), hence high LREE/HREE (Ce/Yb >40) compared to many well-studied high-silica rhyolite glasses and whole-rock compositions. Most minerals record some trace element heterogeneities, with, in particular, one large hornblende phenocryst showing four- to six-fold core-to-rim increases in Sr and Ba coupled with a decrease in Sc. The depletions of Y and HREE in the Fish Canyon glass relative to the whole-rock composition (concentrations in glass ~30% of those in whole rocks) reflect late crystallization of phases wherein these elements were compatible. As garnet is not stable at the low-P conditions at which the Fish Canyon magma crystallized, we show that a combination of modally abundant hornblende (~4%) + titanite (~0.5–1%) and the highly polymerized nature of the rhyolitic liquid led to Y and HREE depletions in melt. Relatively high Sr and Ba contents in glass and rimward Sr and Ba increases in euhedral, concentrically zoned hornblende suggest partial feldspar dissolution and a late release of these elements to the melt as hornblende was crystallizing, in agreement with textural evidence for feldspar (and quartz) resorption. Both observations are consistent with thermal rejuvenation of the magma body prior to eruption, during which the proportion of melt increased via feldspar and quartz dissolution, even as hydrous and accessory phases were crystallizing. Sr/Y in Fish Canyon glass (13–18) is lower than the typical adakitic value (>40), confirming that high Sr/Y is a reliable indicator of high-pressure magma generation and/or differentiation wherein garnet is implicated.     

Development of the Long Valley,California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain

Glass Mountain, California, consists of >50 km³ of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of >1 my preceding explosive eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These “minimum-melt” rhyolites yield Fe-Ti-oxide temperatures of 695–718°C and contain sparse phenocrysts of plagioclase+quartz+magnetite+apatite±sanidine, biotite, ilmenite, allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than in the younger lavas (1.2–0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb, suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone; rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single chamber tapped by all lavas was small enough that the composition of its upper reaches easily affected by new additions of crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas as reflecting eruptions from a large, integrated magma chamber. The small number of cruptions between 1.4 and 1.2 ma may have allowed time for a large magma body to coalesce, and, as the chamber grew, its upper reaches became less affected by new inputs of crustal melts, so that trace-element trends in magmas erupted after 1.2 Ma are largely controlled by fractional crystallization. The extremely low Sr concentrations of Glass Mountain lavas imply extensive crystallization in chambers at least hundreds of cubic kilometers in volume. The close similarity in Sr, Nd, and Pb isotopic ratios between the younger Glass Mountain lavas and unaltered Bishop Tuff indicates that they tapped the same body of magma, which had become isotopically homogenous by 1.2 Ma but continued to differentiate after that time. From 1.2 to 0.79 Ma, volumetric eruptive rates may have exceeded rates of differentiation, as younger Glass Mountain lavas become slightly less evolved with time. Early-erupted Bishop Tuff is more evolved than the youngest of the Glass Mountain lavas and is characterized by slightly different trace element ratios. This suggests that although magma had been present for 0.5 my, the composiional gradient exhibited by the Bishop Tuff had not been a long-term, steady-state condition in the Long Valley magma chamber, but developed at least in part during the 0.06-my hiatus between extrusion of the last Glass Mountain lava and the climactic eruption.     

The evolution of a silicic magma system: isotopic and chemical evidence from the Woods Mountains volcanic center,eastern California

The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have _Nd values of -6.2 to -7.5 and initial ⁸⁷Sr/⁸⁶Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the _Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the ⁸⁷Sr/⁸⁶Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO₂ contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.     

Oxygen isotope thermometry in carbonatites, Fuerteventura, Canary Islands, Spain

Summary Crystallization temperatures of the oceanic carbonatites of Fuerteventura, Canary Islands, have been determined from oxygen isotope fractionations between calcite, silicate minerals (feldspar, pyroxene, biotite, and zircon) and magnetite. The measured fractionations have been interpreted in the light of late stage interactions with meteoric and/or magmatic water. Cathodoluminescence characteristics were investigated for the carbonatite minerals in order to determine the extent of alteration and to select unaltered samples. Oxygen isotope fractionations of minerals of unaltered samples yield crystallization temperatures between 450 and 960°C (average 710°C). The highest temperature is obtained from pyroxene–calcite pairs. The above range is in agreement with other carbonatite thermometric studies.This is the first study that provides oxygen isotope data coupled with a CL study on carbonatite-related zircon. The CL pictures revealed that the zircon is broken and altered in the carbonatites and in associated syenites. Regarding geological field evidences of syenite–carbonatite relationship and the close agreement of published zircon U/Pb and whole rock and biotite K/Ar and Ar–Ar age data, the most probable process is early zircon crystallization from the syenite magma and late-stage reworking during magma evolution and carbonatite segregation. The oxygen isotope fractionations between zircon and other carbonatite minerals (calcite and pyroxene) support the assumption that the zircon would correspond to the early crystallization of syenite–carbonatite magmas.     

Oxygen isotope thermometry of basic lavas and mantle nodules

Measurements have been made of the oxygen isotope and chemical composition of glass and phenocrysts in lavas and coexisting minerals in mantle nodules. Temperatures of formation of these assemblages have been estimated from various chemical thermometers and range from 855° to 1,300° C. The permil fractionations between coexisting orthopyroxene and clinopyroxene in the lavas and nodules are all near zero. The fractionations between pyroxene and olivine vary from +1.2 to –1.4 and are a smooth function of temperature over the entire range. This function is given by T(° C)=1151-173 (px-d)-68²(px-d) and has an uncertainty of ±60° (2). At temperatures above 1,150° C, olivine in the nodules becomes more¹⁸O-rich than coexisting clinopyroxene, orthopyroxene, and plagioclase. In combination with the experimental work of Muehlenbachs and Kushiro (1974), the olivine-pyroxene fractionations indicate that olivine also becomes substantially more¹⁸O-rich than basaltic liquids above 1,200° C. Geothermometers based on the oxygen isotope equilibration of basaltic liquid with olivine, pyroxene, and plagioclase are presented.     

Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics

In this study, we present ⁸⁷Rb/⁸⁶Sr and ²³⁰Th/²³⁸U isotope analyses of glasses and phenocrysts from postcaldera rhyolites erupted between 150 to 100 ka from the Long Valley magmatic system. Both isotope systems indicate complex magma evolution with preeruptive mineral crystallization and magma fractionation, followed by extended storage in a silicic magma reservoir. Glass analyses yield a Rb-Sr isochron of 257 ± 39 ka, which can be explained by a feldspar-fractionation event ∼150 ky before eruption. Individual feldspar-glass pairs confirm this age result. A mineral ²³⁰Th-²³⁸U isochron in a low-silica rhyolite from the Deer Mountain Dome defines an age of 236 ± 1 ka, but the glass and whole rock do not lie on the isochron. U-Th fractionation of the rocks is controlled by the accessory minerals zircon and probably allanite, which crystallized at 250 ± 3 ka and 187 ± 9 ka, respectively. All major mineral phases contain accessory mineral phases; therefore, the mineral isochron represents a mixture of zircon and allanite populations. A precision of ±1 ka for the mixing array implies that the minor phases must have crystallized within this timescale. Longer periods of crystal growth would cause the mixing array to be less well defined. U-series data from other low- and high-silica rhyolites indicate younger accessory mineral crystallization events at ∼200 and 140 ka, probably related to the thermal evolution of the magma reservoir. These crystallization events are, however, only documented by the accessory minerals and had no further influence on bulk magma compositions. We interpret the indistinguishable age results from both isotope systems (∼250 ka) to record the fractionation of small magma batches by filter pressing from a much larger underlying magma volume, followed by physical isolation and extended storage at the top of the magma reservoir for up to 150 ky.     

Boron isotopic variations in hydrous rhyolitic melts: a case study from Long Valley,California

In this paper, we present boron isotope analyses of variably degassed rhyolitic glasses from Long Valley, California. The following results indicate that pre-eruptive boron isotopic signatures were preserved in degassed glasses: (1) averaged secondary ionization mass spectrometry (SIMS) measurements of H₂O-rich (~3 wt%) melt inclusions from late erupted Bishop Tuff pumice are indistinguishable from positive thermal ionization mass spectrometry (PTIMS) analysis of vesiculated groundmass glass (¹¹B=+5.0±0.9 and +5.4±5, respectively); (2) SIMS spot-analyses on H₂O-poor obsidian (~0.15 wt% H₂O) from younger Glass Mountain Dome YA (average ¹¹B=+5.2±1.0) overlap with compositionally similar late Bishop Tuff melt inclusions; and (3) four variably degassed obsidian samples from the 0.6 ka Mono Craters (H₂O between 0.74 and 0.10 wt%) are homogeneous with regard to boron (average ¹¹B=+3.2±0.8, MSWD=0.4). Insignificant variations in ¹¹B between early and late Bishop Tuff melt inclusion glasses agree with published experimental data that predict minor ¹¹B depletion in hydrous melts undergoing gas-saturated fractional crystallization. Melt inclusions from two crystal-rich post-caldera lavas (Deer Mountain and South Deadman Dome) are comparatively boron-rich (max. 90 ppm B) and have lower ¹¹B values (average ¹¹B=+2.2±0.8 and –0.4±1.0 ) that are in strong contrast to the boron isotopic composition of post-caldera crystal-poor rhyolites (27 ppm B; ¹¹B=+5.7±0.8). These variations in ¹¹B are too large to be caused by pre-eruptive degassing. Instead, we favor assimilation of ¹¹B depleted low-temperature hydrothermally altered intrusive rocks subsequent to fresh rhyolite recharge.Editorial responsibility: J. HoefsAn erratum to this article can be found at     

Empirical tests of carbon isotope thermometry in granulites from southern California

Abstract ‘Peak’metamorphic carbon isotope fractionations between calcite and graphite (Δ_Cal–Gr) in marbles and calc-silicates from the Cucamonga granulite terrane (San Gabriel Mountains, California) range from 3.48 to 2.90%. The data are used to test three previously published calibrations of the calcite–graphite carbon isotope thermometer. An empirical calibration of the calcite–graphite carbon isotope thermometer gives temperatures of 700–750°C; a theoretical–experimental calibration of the system gives temperatures of 760°–870°C; an experimental calibration gives temperatures of 870–1300°C. Temperatures calculated using the empirical calibration are in agreement with those calculated from garnet-based cation exchange thermometry when uncertainty is considered. Temperatures calculated using the theoretical–experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50°C higher. The experimental calibration yields temperatures from 50 to 480°C higher than those from cation exchange thermometry. Moreover, temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage. Despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite–graphite system, temperatures calculated using the theoretical–experimental calibration may be real peak metamorphic temperatures. If retrograde diffusion partially reset garnet-based cation exchange thermometers by c. 50°C, then the cation exchange temperatures are consistent with those from the theoretical–empirical calibration. Thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid-present, or the theoretical–experimental calibration if melting was fluid-absent.     

岩浆补给作用对硅质火山岩浆系统演化的制约

硅质火山喷发作为大陆地壳岩浆活动的重要表现，在研究大陆地壳形成与演化、探讨岩浆过程与动力学机制等方面具有重要的价值，其通常所表现的强烈爆炸式喷发，甚至可以导致全球性的环境和气候变迁。硅质岩浆系统在开放体系中不同来源岩浆的贡献和相互作用是目前研究的热点问题。持续的岩浆补给可以延长岩浆存储的时间，促进岩浆房的对流、岩浆的分异演化以及晶体 熔体的分离和晶粥的再活化，同时也是触发火山喷发的重要机制之一。此外，岩浆补给以及硅质岩浆的晶体 熔体演化过程也是火山喷发产物多样性的原因，导致同一火山在其活动过程中喷发产物规律性的变化，如富晶体火山岩、贫晶体火山岩、火山岩成分分层、以及复活岩穹和中央侵入体等。因此，岩浆补给作用是制约硅质火山岩浆系统演化和火山岩成分多样性的重要因素，也是活动火山监测和灾害评估的重要依据。岩石学、岩石地球化学、矿物(长石、石英、石榴子石、锆石等)同位素及成分变化，以及模拟实验、地震层析成像等研究为揭示硅质岩浆系统中的岩浆补给作用和复杂岩浆过程提供了多种视角。     

Nd,Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera,Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km³), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial _Nd values (1 _Nd unit), higher⁸⁷Sr/⁸⁶Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar _Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher _Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low _Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The low _Nd magmas most likely formed via the incorporation of low ¹⁸O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher ¹⁸O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low _Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing basification of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.     

Oxygen,hydrogen and carbon isotope studies of the franciscan formation,Coast Ranges,California

Analyses of 230 Franciscan rock and mineral samples, including the San Luis Obispo ophiolite, show that metamorphism produces no change in the δ¹⁸O of the graywackes (+11 to +14), but that igneous rocks become enriched in ¹⁸O by 2–6% and the cherts depleted by 5–10%. The shales are of two types, a high-¹⁸O type (+16 to +20) associated with chert and a low-¹⁸O type isotopically and mineralogically similar to the graywackes. The vein quartz (δ = + 15 to + 20) is invariably richer in ¹⁸O than the host rock quartz and in most of the rocks the δ¹⁸O of the clastic quartz is similar to the δ¹⁸O of the whole rock. Mineral assemblages are typically not in isotopic equilibrium. Although the δ¹⁸O values are very uniform (+13 to +16). the δ¹³C of vein aragonite and calcite is widely variable (0 to ? 14), implying that a major source of the carbon is oxidized organic material. The δD values of 83 igneous and sedimentary rocks are -45 to -80, exceptions are the Fe-rich minerals howieite and deerite, which have δD = ?100. All of these samples could have equilibrated with H₂O having δD ≈ +10 to ?20 and δ¹⁸O ≈ ?3 to +8. assuming temperatures of 100–300°C. However, the serpentines (δD ≈ ?85 to ?110) and the vein minerals (δD = ?23 to ?55) are exceptions. The vein minerals are 10–20%, richer in deuterium than the adjacent wall rocks; they formed from a relatively D-rich metamorphic water, typically at lower temperatures than did their host rocks. The isotopic compositions of the other Franciscan rocks were affected by three distinct events: (1) hydrothermal alteration of the ophiolite complexes and volcanic rocks as a result of submarine igneous activity at a spreading center or in an island-arc environment; (2) low-temperature, high-pressure regional metamorphism and diagenesis; and (3) a late-stage, very low temperature (<100°C) alteration of the ultramafic bodies by meteoric ground waters, producing lizardite-chrysotile serpentine. In the first two cases, the pore fluid involved in the alteration of the Franciscan rocks was sea water. However, this water became somewhat depleted in D and enriched in ¹⁸O during blueschist metamorphism, evolving to values of δD ≈ ? 20 and δ¹⁸O ≈ + 6 to + 8 at the highest grades. Except for one graywacke sample, the meteoric waters that affected the serpentinites did not significantly change the $\text{D}\text{H}$ ratios of the OH-bearing minerals in any other Franciscan rock.The δ¹⁸O values of orogenic andesites are too low for such magmas to have formed by direct partial melting of Franciscan-type materials in a subduction zone. Andesites either form in some other fashion, or the melts must undergo thorough isotopic exchange with the upper mantle. The great Cordilleran granodiorite-tonalite batholiths, however, are much richer in ¹⁸O and may well have formed by large-scale melting or assimilation of Franciscan-type rocks. The range of δD values of Franciscantype rocks is identical to the ?50 to ?80 range shown by most igneous rocks. This suggests that ‘primary magmatic H₂O’ throughout the world may be derived mainly by partial melting of Franciscantype materials, or by dehydration of such rocks in the deeper parts of a Benioff zone.     

Lead isotopes by LA-MC-ICPMS: Tracking the emergence of mantle signatures in an evolving silicic magma system

At Long Valley (LV) model Sr isotope phenocryst ages and absolute U-Pb zircon ages from precaldera Glass Mountain (GM) and caldera-related Bishop Tuff (BT) rhyolites show that these crystals track >1 Myr of evolution of a voluminous rhyolite magmatic system. In detail, strong disparities between the different age populations complicate ideas for a unified model for rhyolite generation, differentiation, and storage. To better elucidate the age discrepancies a new in situ Pb isotope technique has been developed to measure the compositions of 113 individual LV feldspars (mainly sanidine) and their host glasses by UV laser ablation MC-ICPMS. Given sufficient signal the accuracy and precision of this technique approaches that of double-spike thermal ionization mass spectrometry. The utility of our technique for many geologic materials is, however, limited to determining Pb isotope ratios that include ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb, but exclude ²⁰⁴Pb. New zircon ²³⁸U-²⁰⁶Pb crystallization ages were also obtained for two older Glass Mountain domes.A >1.5‰ difference between the Pb isotope compositions of feldspars from older (1.7-2.2 Ma) precaldera Glass Mountain (GM) rhyolites and younger LV rhyolites, including the BT, is found. The Pb isotope data for feldspars and their host glasses lie along a regional trend line between young basalts and evolved crust compositions, spanning ∼15% of that isotopic difference, and show a secular change towards increasing mantle contribution. Most feldspars have Pb isotope compositions that are similar to their host glasses and, as such, there persists an apparent >100 k.y. difference between Sr model feldspar ages and zircon ages for some GM rhyolites. Collectively, the feldspars define a Sr-Pb isotope mixing curve. Evidence for mixing complicates the interpretation that the Sr isotope data solely reflect radiogenic ingrowth. Where isotopically heterogeneous feldspar populations occur, there is greater uncertainty about the veracity of the Sr model ages. Specifically, we find no Pb isotope evidence that BT feldspars grew from older GM-like magmas.The distinct Pb isotope signatures for individual rhyolites and their feldspars support evidence based on zircon dating that LV volcanism did not erupt from a single long-lived magma chamber but rather tapped a number of different magmas. Moreover, contrary to the conventional model of gradual build-up prior to cataclysmic eruption, secular changes in the U-Pb age constraints on magma residence times and the magmas’ distinct Pb isotopic compositions suggest that, at Long Valley, eruptive volumes increase with shorter magma residence time and correlate with greater mantle input. Evidently, the plumbing and therefore activity at Long Valley was influenced by the evolving interaction between source and crustal magma system.     

石英一水一盐体系氧同位素分馏作用

温度为180—550℃,盐度(wt.％)分别为0、5、25和40条件下,在高压釜内完成了由硅胶合成石英的氧同位素分馏作用实验研究,目的是了解:①盐同位素效应;②△t值对同位素分馏的影响;③温度与同位素分馏系数的关系。研究资料表明:低温条件下矿物和纯水之间同位素平衡作用不可能发生;影响含氧矿物(初)之间氧同位素平衡速率的因素包括盐度、△t值大小和温度等;我们的研究还表明,盐度对同位素分馏作用同系数无影响,即不存在所谓的“同位素盐效应”。在180—550℃温度范围内,不同盐度条件下获得的石英-水氧同位素分馏实验方程为:10001nα_(石英-水)=3.306×10~5T~(-2)—2.71。     

The origin and significance of large,tabular dunite bodies in the Trinity peridotite,northern California

Kilometer-sized, tabular dunite bodies are contained within harzburgite, lherzolite and plagioclase lherzolite host rocks in the Trinity peridotite, northern California. An igneous origin for the dunite by crystal fractionation of olivine from a melt is suggested by their tabular shapes, clots of poikilitic clinopyroxene grains, chromite pods, and by analogy to dunite bodies in the Samail and Vourinos ophiolites (Hopson et al. 1981; Harkins et al. 1980; Moores 1969). However, structures and systematic variations in mineralogy and mineral chemistry suggest that at least the marginal few meters of the bodies are residues produced by extraction of a basaltic component from a plagioclase lherzolite protolith. A model is suggested in which a picritic melt ascended through the upper mantle in vertically oriented channels. Part of the dunite in the tabular bodies was produced by fractional crystallization of olivine from the melt. Additional dunite at the margins of the bodies was formed by extraction of a basaltic component from plagioclase lherzolite wall-rocks during partial assimilation by the picritic melt. The latter process is similar to the wall-rock reaction discussed by Green and Ringwood (1967) and is essentially zone refining of the the mantle wall rocks by the migrating melt. It is significant because it suggests a mechanism in addition to fractional crystallization for enrichment of incompatible elements in basalts.Submitted to Contributions to Mineralogy and Petrology, April, 1981. Resubmitted after Review, October, 1981     

Rapid open-system assembly of a large silicic magma body: time-resolved evidence from cored plagioclase crystals in the Oruanui eruption deposits, New Zealand

Rhyolitic pumices in the 26.5 ka Oruanui eruption (Taupo volcano, New Zealand) contain an average of 10 wt% crystals. About 2 wt% of the crystal population is feldspar crystals that display bluish–grey cloudy cores, the colour being imparted by exsolved needles of rutile. The volume of cloudy-cored feldspars thus amounts to ~1.0 km³ in a total magma volume of ~530 km³. The cored feldspars show great variability in detail, but in general have a rounded cloudy core bounded by a zone rich in glass and mineral inclusions, that was then overgrown by a euhedral clear rim. Sr-isotopic variations in eight representative crystals were measured on micromilled samples of selected growth zones in the cores and rims, and linked to feldspar compositions through microprobe traverses. The cloudy cores range from ⁸⁷Sr/⁸⁶Sr = 0.70547 to 0.70657, with compositions of An₄₃ to An₇₈. The overgrowth rims display wider variations: inner parts show extreme ranges in composition (maxima ⁸⁷Sr/⁸⁶Sr = 0.70764 and An₇₈), while outer parts in seven of eight crystals are zoned, with outward-decreasing Sr-isotopic and An values to figures that are in accord with the bulk pumice and other, clear-feldspar values, respectively. The three parts of the crystals represent distinct regimes. The cloudy cores are inherited from an intermediate plutonic protolith that has been subjected to melting. The inner overgrowth rims were crystallised from a high temperature, relatively radiogenic melt derived from Mesozoic-Palaeozoic metasedimentary rocks (“greywacke”). The outer euhedral rims reflect mixing into and continued growth within the growing Oruanui magma body. The cloudy-cored feldspars also contain rare zircon inclusions. Twenty one zircons were recovered by HF digestion of a bulk sample of cloudy feldspars and analysed by SHRIMP for U–Th isotopes with which to calculate model ages. Eighteen of 21 crystals returned finite ages, the model-age spectrum of which is similar to the age spectra from free zircons in Oruanui pumices. Assembly of the Oruanui magma body was not only rapid (over ~40 kyr, as shown by other data) but involved a wide open system, with significant contributions from partly-melted intermediate-composition igneous intrusions (cloudy cores) and greywacke melts (inner overgrowths) being introduced into the magma body up to the point of eruption. Such open system behaviour contrasts with that proposed in models for comparably voluminous silicic magmas derived dominantly by fractionation (such as the Bishop Tuff) where the magma and its crystal cargo were better insulated thermally and chemically from country-rock interaction.     

A model of reverse differentiation at Dikii Greben' Volcano,Kamchatka: progressive basic magma vesiculation in a silicic magma chamber

Dikii Greben' Volcano is the largest modern volcano with silicic rocks in the Kurile-Kamchatka island arc. It consists of many domes and lava flows of rhyodacite, dacite and andesite which were erupted in a reverse differentiation sequence. Non-equilibrium phenocryst assemblages (quartz + Mg-rich olivine, An-rich + An-poor plagioclase etc.), abundance of chilled mafic pillows in the dacites and andesites, and linear variations of rock compositions in binary plots are considered as mineralogical, textural and geochemical evidence for mixing. Mafic pillows in volcanics have a lower density (because of high porosity) and contain the same non-equilibrium phenocryst assemblages as the host rocks. Their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water-rich basaltic magma at depth. They are considered as supercooled, vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling. The lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption. Magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption. The rhyodacitic and basic end-members of the mixing series cannot be linked by low-P fractionation though high-P, amphibole-rich fractionation is not excluded.     

Oxygen isotope study of the Suzhou granite

Oxygen isotope studies of the Suzhou granite have been made based on drill-hole samples. In the inner part, the δ¹⁸O values are less variable either in the whole-rock, quartz or in feldspar. Oxygen isotopic compositions are in equilibrium between coexisting quartz and feldspar. Data points from the inner part are plotted in a small area in the δ¹⁸O_Q-δ¹⁸O_f diagram, indicating that this part has not been affected by meteoric hydrothermal fluids. But the whole-rock δ¹⁸O values of the marginal part vary greatly. Oxygen isotopic compositions are in extreme disequilibrium between quartz and feldspar. Data points from the marginal part are displayed with a nearly vertical slope in the δ¹⁸O_Q-δ¹⁸O_f diagram, implying that rocks of this part are affected by the meteoric hydrothermal fluids. Extreme water-rock interactions lead to mineralizations of rare-elements (Nb, Ta, as well as Zr, Hf, Th) in the marginal part. Source materials of the Suzhou granite are also discussed in this paper.     

Rapid pre-eruptive thermal rejuvenation in a large silicic magma body: the case of the Masonic Park Tuff,Southern Rocky Mountain volcanic field,CO, USA

Determining the mechanisms involved in generating large-volume eruptions (>100 km³) of silicic magma with crystallinities approaching rheological lock-up (~50 vol% crystals) remains a challenge for volcanologists. The Cenozoic Southern Rocky Mountain volcanic field, in Colorado and northernmost New Mexico, USA, produced ten such crystal-rich ignimbrites within 3 m.y. This work focuses on the 28.7 Ma Masonic Park Tuff, a dacitic (~62–65 wt% SiO₂) ignimbrite with an estimated erupted volume of ~500 km³ and an average of ~45 vol% crystals. Near-absence of quartz, titanite, and sanidine, pronounced An-rich spikes near the rims of plagioclase, and reverse zoning in clinopyroxene record the reheating (from ~750 to >800?°C) of an upper crustal mush in response to hotter recharge from below. Zircon U–Pb ages suggest prolonged magmatic residence, while Yb/Dy vs temperature trends indicate co-crystallization with titanite which was later resorbed. High Sr, Ba, and Ti concentrations in plagioclase microlites and phenocryst rims require in-situ feldspar melting and concurrent, but limited, mass addition provided by the recharge, likely in the form of a melt-gas mixture. The larger Fish Canyon Tuff, which erupted from the same location ~0.7 m.y. later, also underwent pre-eruptive reheating and partial melting of quartz, titanite, and feldspars in a long-lived upper crustal mush following the underplating of hotter magma. The Fish Canyon Tuff, however, records cooler pre-eruptive temperatures (~710–760?°C) and a mineral assemblage indicative of higher magmatic water contents (abundant resorbed sanidine and quartz, euhedral amphibole and titanite, and absence of pyroxene). These similar pre-eruptive mush-reactivation histories, despite differing mineral assemblages and pre-eruptive temperatures, indicate that thermal rejuvenation is a key step in the eruption of crystal-rich silicic volcanics over a wide range of conditions.     

Compositional gradients in large reservoirs of silicic magma as evidenced by ignimbrites versus Taylor Creek Rhyolite lava domes

The Taylor Creek Rhyolite of southwest New Mexico consists of 20 lava domes and flows that were emplaced during a period of a few thousand years or less in late Oligocene time. Including genetically associated pyroclastic deposits, which are about as voluminous as the lava domes and flows, the Taylor Creek Rhyolite represents roughly 100 km³ of magma erupted from vents distributed throughout an area of several hundred square kilometers. Major-element composition is metaluminous to weakly peraluminous high-silica rhyolite and is nearly constant throughout the lava field. The magma reservoir for the Taylor Creek Rhyolite was vertically zoned in trace elements, ⁸⁷Sr/⁸⁶Sr, and phenocryst abundance and size. Mean trace-element concentrations, ranges in concentrations, and element-pair correlations are similar to many subalkaline silicic ignimbrites. However, the polarity of the zonation was opposite that in reservoirs for ignimbrites, for most constituents. For example, compared to the Bishop Tuff, only ⁸⁷Sr/⁸⁶Sr and Sc increased upward in both reservoirs. Quite likely, a dominant but nonerupted volume of the magma reservoir for the Taylor Creek Rhyolite was zoned like that for the Bishop Tuff, whereas an erupted, few-hundred-meter-thick cap on the magma body was variably contaminated by roof rocks whose contribution to this part of the magma system moderated relatively extreme trace-element concentrations of uncontaminated Taylor Creek Rhyolite but did not change the sense of correlation for most element pairs. The contaminant probably was a Precambrian rock of broadly granitic composition and with very high ⁸⁷Sr/⁸⁶Sr. Although examples apparently are not yet reported in the literature, evidence for a similar thin contaminated cap on reservoirs for large-volume silicic ignimbrites may exist in the bottom few meters of ignimbrites or perhaps only in the pumice fallout that normally immediately precedes ignimbrite emplacement. ⁸⁷Sr/⁸⁶Sr in sanidine phenocrysts of the Taylor Creek Rhyolite is higher than that of their host whole rocks. Covariation of this isotope ratio with sanidine abundance and size indicates positive correlations for all three features with decreasing distance to the roof of the magma reservoir. The sanidine probably is more radiogenic than host whole rock because growing phenocrysts partly incorporated Sr from the first partial melt of roof rocks, which contained the highly radiogenic Sr of Precambrian biotite ± hornblende, whereas diffusion was too slow for sanidine to incorporate much of the Sr from subsequently produced less radiogenic partial melt of roof rocks, before eruption quenched the magma system. Disequilibrium between feldspar phenocrysts and host groundmass is fairly common for ignimbrites, and a process of contamination similar to that for the Taylor Creek Rhyolite may help explain some of these situations.