Open-system Processes Research Articles

Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ57Fe are in a range of −0.25 to 0.14‰ for olivine, −0.17 to 0.17‰ for orthopyroxene, −0.21 to 0.27‰ for clinopyroxene, and −0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.

Dimensions of Performance Measurement System in Changes Research

Research in management accounting and performance measurement is eclectic. None of the conceptual approaches dominate. Some of them, such as contingency, agents, contract theories are popular among management accounting researchers, others are quite new at least in the sense of application in performance measurement studies, namely, complexity and open systems theory. Analysis of performance measurement from different theoretical points of view leads to the formulation of framework with featured theoretical dimensions. Elements of performance measurement systems interpreting it as an open system are presented in this study. Two pure theoretical situations of performance measurement system are determined.Performance measurement could be objectively based on hard measures and their interpretation (explicit knowledge) or subjectively based on experience, analytical abilities and intuition (tacit knowledge). Both of them could be disclosed in each organization but in different levels. Those aspects could be analyzed according to the featured elements of open system (input, process and output) and forms of perfection level of a particular organization. Dimensions of theoretical framework as well as elements of open system could be the basis constructing instruments of empirical performance measurement researches.This paper aims to disclose how changes of organizational environment are reflected in performance measurement system.Experience in this field is unique in each organization. In most cases quantitative research methods have some limitations to disclose and to generalize this experience. Changes of case study organization’s performance system’s content were analyzed during three periods. Performance systems content were disclosed using research framework based on open systems dimensions. Organizational values were chosen as a measure of organizational environment. Some implications to economic conditions as shaping factors for organizational environment were made.The main results are findings that economic conditions could be enabling factor for the unique configuration of objective and subjective features of open performance measurement system in each organization. This impact could be analyzed indirectly through organizational values and reflection of them in performance measurement system’s perfection level. Interpretations of those results do not lead to generalized conclusions but shed more light on the phenomena of performance measurement system and defines the way it could be diagnosed, interpreted and improved.

Minerals, Inclusions and Volcanic Processes: KEITH D. PUTIRKA and FRANK J. TEPLEY III, Editors. Pp. 674. 2008. Reviews in Mineralogy and Geochemistry, v. 69, 2008. ISBN 978-0-939950-83-6. US$40.00.

This book is volume 69 in the long list of Reviews in Mineralogy and Geochemistry publications produced jointly by the Mineralogical Society of America and the Geochemical Society. It is composed of 17 chapters, including an introductory chapter, one on experimental studies, several on thermometers and barometers, and others on petrology, magma ascent rates, melt inclusions, volatiles, isotopes, and mineral textures and zoning. This volume summarizes volcanologic and magmatic processes related to minerals, and how advances made in microanalytical techniques have impacted on the study of those processes. The composition of minerals commonly leads to insights not easily gained from whole-rock analyses. Indeed, minerals and their inclusions can provide information on magma compositions, rates of magma ascent, open-system processes, magma hybridization, and pressure and temperature conditions. The chapters in this book are laid out to essentially reflect these parameters. Chapter 2 begins by tracing the origins of mineral grains and methods to estimate pressures and temperatures of crystallization. An emphasis is placed on crystallization …

Coupling Effect Between Coalification and Tectonic-thermal Events—Geological Records of Geodynamics of Sedimentary Basin

Coal is an organic sedimentary rock, which is very sensitive to geological factors, such as temperature and pressure. Coalification is actually an open system process in which coal exchanges energy and substance with the surrounding environment. During the long process of geologic evolution, all types of tectonic-thermal activities resulted in the diversification of coal in terms of physics, chemistry, textures, and structures. Affected by various geologic agents, the organic and inorganic components in coal moved, differentiated, and rebuilt, which sequentially influenced the coal seam distribution, coal rank, and coal quality, which can be treated as the geological record of the geodynamics of sedimentary basin to some degree. In this article, research progresses of coupling effects between coalification and tectonic-thermal events at home and abroad are introduced, the main scientific problems are discussed, and some key research projects are proposed.

Generalized Kramers-Heisenberg expressions for stimulated Raman scattering and two-photon absorption

The frequency-domain pump-probe signal of a material system interacting with two quantum modes of the radiation field is recast in terms of products of scattering amplitudes (T matrix elements) rather than the third-order susceptibility Im chi((3)). The resulting expression offers a more intuitive physical picture for the optical process compared with the semiclassical approach which treats the radiation field as classical. It can be derived and interpreted using closed-time-path-loop diagrams which represent the joint state of the matter and the field for each contribution to the signal. The signal has two components representing stimulated Raman scattering omega(1) - omega(2) and two-photon absorption omega(1) + omega(2) two-photon resonances. Both are expressed as nonequi-librium steady-state photon and matter fluxes, as is common in the description of dissipative processes in open quantum systems.

Semiclassical theory for decay and fragmentation processes in chaotic quantum systems

We consider quantum decay and photofragmentation processes in open chaotic systems in the semiclassical limit. We devise a semiclassical approach which allows us to consistently calculate quantum corrections to the classical decay to high order in an expansion in the inverse Heisenberg time. We present results for systems with and without time-reversal symmetry, as well as for the symplectic case, and extend recent results to nonlocalized initial states. We further analyze related photodissociation and photoionization phenomena and semiclassically compute cross-section correlations, including their Ehrenfest-time dependence.

Intermediate composition magma production in an intracontinental setting: Unusual andesites and dacites of the mid-Miocene Santa Rosa–Calico volcanic field, Northern Nevada

The mid-Miocene Santa Rosa–Calico volcanic field (SC) of northern Nevada provides an outstanding example of the role open-system magmatic processes play in producing calc-alkaline and tholeiitic andesite–dacite magmas in an intracontinental setting. SC volcanism commenced at ∼ 16.7 Ma and is associated with the initial manifestations of the Yellowstone hotspot, the Columbia River–Steens flood basalt event(s), and the formation of the Northern Nevada rift. Locally a diverse package of magmatic products ranging from tholeiitic basalt to high-Si rhyolite was produced during an ∼ 2 myr duration. Within this package are the products of at least four distinct intermediate composition magmatic systems that may represent as much as 40% of the SC volcanic pile. These help differentiate the SC from contemporaneous Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee, Northwest Nevada) that are dominated by bimodal basalt–rhyolite assemblages. All SC intermediate units are characterized by textural and mineralogic complexities including xenoliths and xenocrysts of local crust and crystal clots of plagioclase ± clinopyroxene ± orthopyroxene ± oxide. SC intermediate units are dominantly tholeiitic, but include lava flows with transitional to calc-alkaline affinities. Relative to locally erupted Steens Basalt, SC intermediate lava flows have similar elemental enrichments and depletions, but dissimilar Sr and Nd isotopic compositions. These isotopic differences, coupled with the abundant disequilibrium features and variable incompatible element ratios, indicate that open system magmatic processes played a major role in the genesis of the intermediate units. SC silicic magmas were produced primarily via upper crustal melting of chemically and isotopically heterogeneous Cretaceous granitoid. Interaction between fractionating mafic Steens flood basalt magmas and the more evolved crustal melts ± assimilation of local upper crust provides a general template for the generation of SC tholeiitic and calc-alkaline intermediate magmas. Between system heterogeneities primarily reflect magma- versus assimilation-dominated mixing. These complex processes and the resulting production and eruption of SC intermediate composition melts were stimulated by incipient, focused lithospheric extension and driven by the virtually continuous local availability of upwelling Steens Basalt, over at least a 2 myr duration. These results demonstrate that at least some of the mid-Miocene and younger calc-alkaline magmas that are found across the Pacific Northwest and are unrelated to subduction processes, were formed as a direct result of open-system processes affecting regional flood basalt magmas, rather than just melting of previously enriched (via subduction) lithospheric mantle as suggested by previous workers.

The Eocene bimodal Piranshahr massif of the Sanandaj–Sirjan Zone, NW Iran: a marker of the end of the collision in the Zagros orogen

Abstract: The bimodal Piranshahr massif is composed of coeval but geochemically unrelated mafic (40.7 ± 0.2 Ma zircon U–Pb sensitive high-resolution ion microprobe (SHRIMP) age) and A-type felsic rocks (41 ± 0.5 Ma Rb–Sr and 41.3 ± 0.8 Ma zircon U–Pb SHRIMP age). The mafic rocks consist of two geochemical types of gabbros that derived from different magmas. The more abundant gabbros are moderately alkaline, have ratios of large ion lithophile elements to REE and high field strength elements to REE similar to those of intraplate mantle magmas, 87 Sr/ 86 Sr 41 Ma ≈ 0.7036 and ε(Nd) 41 Ma ≈ +7.2. The less abundant gabbros have calc-alkaline affinities, 87 Sr/ 86 Sr 41 Ma ≈ 0.7043 and ε(Nd) 41 Ma ≈ +4.78. Felsic rocks are metaluminous A2-type annite–fayalite–hedenbergite, hypersolvus, leucocratic, coarse-grained agpaitic syenites, pulaskites and granites, with 87 Sr/ 86 Sr 41 Ma ≈ 0.7048 and ε(Nd) 41 Ma ≈ +3.6–4.5. Syenites, pulaskites and granites are genetically related. Pulaskites probably represent alkali-enriched water-rich residual melts from which an F-rich vapour phase was later separated. Granites were probably generated during open-system processes, in which F-rich hydrous alkaline fluids released from the syenites acted upon pre-existing felsic rocks. The c . 41 Ma age of the post-collisional Piranshahr massif indicates that the related collision probably occurred at 50–60 Ma (i.e. Late Palaeocene or Early Eocene), thus resolving a much debated question.

PROPAGATION OF SCALE-DEPENDENT EFFECTS FROM RECRUITS TO ADULTS IN BARNACLES AND SEAWEEDS.

The interplay between local and large spatial scale processes in open systems is often dependent upon ecological context and species specific factors such as longevity, dispersal capability, or vulnerability to predation. When disturbance clears patches in open systems, the successful reestablishment of adult colonizers and the trajectory of succession may depend upon both the scale of the disturbance event and scale of life history characteristics. Here we examine the links between the size of a disturbance event and long term patterns of variation in recruitment, density, and percent cover in a relatively short-lived but long-range disperser, the acorn barnacle Semibalanus balanoides, and a relatively long lived and short range disperser, the fucoid alga Fucus vesiculosus. Further, a model of linked scale-dependent processes is developed and we explore how long term patterns of scale-dependent recruitment are related to successful establishment of adults. Recruitment, densities, and cover were monitored for five years for both species in a single experiment using clearings of different sizes spread over two north-facing and two south-facing bays on Swan's Island, Maine, USA. Barnacle recruitment was particularly variable in small clearings and was lower overall at more interior sites within bays. While local-scale factors also strongly influenced patterns of variation in fucoid recruitment, fucoids dominate surface cover in large clearings and in south-facing bays. Recruitment was a poor predictor of density, whereas density itself was a good predictor of percent cover for both species. These results indicate that scale dependencies, location-specific factors, and life history traits contribute to patterns of community development on rocky shores and may ultimately determine whether an open patch converges to or diverges from its initial community state.

Geochemical and B–Sr–Nd isotopic evidence for mingling and mixing processes in the magmatic system that fed the Astroni volcano (4.1–3.8 ka) within the Campi Flegrei caldera (southern Italy)

The Astroni volcano was built through seven eruptions that generated pyroclastic deposits and lava domes within the Campi Flegrei caldera (southern Italy) 4.1–3.8 ka BP. Whole-rock geochemical and B–Sr–Nd isotopic investigations were carried out on representative samples of all seven eruptions. The products vary from tephriphonolites to phonolites, and from latites to trachytes. They show textural, mineralogical and isotopic evidence of disequilibrium, including distinct clinopyroxene populations, rounded and/or resorbed plagioclase and alkali-feldspar, and reverse-zoned phenocrysts of all these mineral phases. The Sr, Nd and B isotopic composition of whole rocks is variable and correlated with the degree of chemical evolution, suggesting open-system processes in addition to fractional crystallisation. Moreover, significant Sr-isotopic disequilibrium between the phenocrysts and glass has been documented for one sample. The chemostratigraphy of the products indicates that Astroni eruptions 1 through 5 were fed by magmas of trachytic to phonolitic composition that were less enriched in radiogenic Sr and 11B up-section. This variability has been interpreted as the result of mingling between at least two distinct magmatic end-members, one more evolved and the other less evolved. Another heterogeneous batch of magma, resulting from almost complete mixing between the same two end-members, was drained during eruptions 6 and 7. The more evolved end-member, characterised by 87Sr/ 86Sr ≥ 0.7075, 143Nd/ 144Nd ≤ 0.51247 and δ 11B ≥ − 8‰, was very similar to the magma that fed the final phases of the Agnano–Monte Spina eruption, which occurred a few centuries earlier in the Astroni vent area. The less evolved end-member had 87Sr/ 86Sr ≤ 0.70726, 143Nd/ 144Nd ≥ 0.51251 and δ 11B ≤ 10‰, and was likely derived by fractional crystallisation of a mantle-derived magma. An abrupt decrease in both the Sr isotope ratio and the Th content, detected at the transition between Unit 4 and 5, suggests that another magma with a 87Sr/ 86Sr ratio intermediate between those of the two identified end-members may have been involved in Astroni activity. The more evolved end-member is interpreted as a residue of the Agnano–Monte Spina eruption that was invaded by either the intermediate or the less evolved magmatic end-member, promoting mingling and triggering Astroni activity. This study of Astroni provides insights for both short- and long-term volcanic hazard assessment, as the Astroni volcano is the best example of a very close sequence of eruptions from the same vent area in the Campi Flegrei caldera.

Processes and Sources during Late Variscan Dioritic–Tonalitic Magmatism: Insights from Plagioclase Chemistry (Gęsiniec Intrusion, NE Bohemian Massif, Poland)

The Ge siniec Intrusion (Strzelin Massif, East Sudetes) ( 307^290 Ma) is composed predominantly of dioritic to tonalitic rocks with Sr/Sr ratios ranging from 0 7069 to 0 7080 and eNd1⁄4 ^3 1 to ^4 2, emplaced as post-collisional magmas following theVariscan orogeny. In situ Sr isotope and trace element analyses of plagioclase were carried out on five diorite^tonalite samples with variable whole-rock Sr/Sr compositions to constrain magma sources and elucidate magma chamber processes. Plagioclase is characterized by complex zoning patterns, such as patchy zoning, asymmetrical zoning and strong resorption between An-rich cores and more albitic rims.The range of Sr isotopic compositions recorded in plagioclase is 0 7069^0 7091, greater than that of the whole-rocks. No change in isotopic composition is observed across resorbed core^ rim boundaries, as would be expected if the resorption was caused by magma mixing.Two samples are close to Sr isotopic equilibrium between plagioclase and the whole-rock. Three samples are isotopically heterogeneous and exhibit the largest variations in Sr isotope ratios within high-An plagioclase cores, associated with only slight resorption and almost no change in An content. Consequently, we interpret the core resorption to be due to decompression during emplacement of phenocryst-bearing magmas in the upper crust rather than to magma mixing, whereas we interpret the isotopic heterogeneity in the high-An plagioclase cores to be due to open-system processes in the lower crust. The trace element and Sr isotopic compositions of the high-An plagioclase cores provide an insight into the lower crustal processes that took place prior to decompression.We show that different diorite^tonalite types cannot be related to each other by assimilation^fractional crystallization processes and consequently they probably evolved as separate magma batches. Processes recorded in plagioclase core compositions include contamination by high Sr/Sr crustal material and interaction with water-rich magma. The studied samples represent magmas that probably formed either by continuous differentiation and crustal contamination of originally more mafic magmas or by remelting of 1 3^2 1 Ga basaltic materials in the lower crust. This study emphasizes the importance of integrating textural and in situ data to elucidate the processes that contribute to the formation of texturally and compositionally complex plagioclase crystals in plutonic rocks.

Volatiles in High-K Magmas from the Western Trans-Mexican Volcanic Belt: Evidence for Fluid Fluxing and Extreme Enrichment of the Mantle Wedge by Subduction Processes

Primitive, high-K minettes and basanites erupted during the Pleistocene from cinder cones on the flanks of the Colima Volcanic Complex in the western Trans-Mexican Volcanic Belt. Melt inclusions in olivine (Fo89–92) from tephra at these cones reveal that both magma types are oxidized and volatile rich, with high H2O (≤6·2 wt%), CO2 (≤5300 ppm), S (≤6700 ppm), Cl (≤2300 ppm), and F (≤8100 ppm) contents. A nearby calc-alkaline basaltic andesite cinder cone with more evolved composition (Fo78–80 olivine) has melt inclusions with similarly high H2O (≤5·5 wt%) but much lower CO2, S, and Cl compared with the potassic magmas. Melt inclusions from each cone have highly variable H2O and CO2, corresponding to crystallization pressures of < 100 bars to ∼ 7 kbar. This indicates that olivine crystallized from variably degassed melts over a wide range of depths extending from the lower crust (>25 km depth) to very shallow levels. The H2O and CO2 variations cannot be explained by simple degassing models but instead requiring more complex, open-system processes or possibly reflect disequilibrium degassing. Trace element variations in the melt inclusions suggest that phlogopite and garnet were residual minerals during melting in the mantle source, and the presence of garnet suggests an origin in asthenospheric rather than lithospheric mantle. Decompression melting of phlogopite–garnet peridotite cannot produce the high H2O contents of the potassic magmas, and thus the presence of fluids during melting is required. Trace element modeling of a mantle source (intermediate in composition between enriched mid-ocean ridge basalt and ocean island basalt sources) that is fluxed with an H2O-rich fluid or hydrous melt from the subducting slab can reproduce most of the trace element characteristics of the potassic melts, demonstrating that they are clearly linked with subduction processes. Formation of the potassic magmas probably involved slab rollback, trenchward migration of the arc into the region above metasomatically enriched forearc mantle, and heating of this veined and fluid-fluxed mantle as a result of upwelling of hot mantle through a tear between the subducted Cocos and Rivera plates.

Sr and Pb Isotope Micro-analysis of Plagioclase Crystals from Skye Lavas: an Insight into Open-system Processes in a Flood Basalt Province

Crystals in many magmatic rocks are heterogeneous in terms of their isotope composition. Detailed crystal-isotope stratigraphy (CIS) studies have shown that crystals act as reliable recorders of their magma source composition and of the pathways of magma interaction, and successfully identify the end-members involved in magmatic systems. In this paper we have analysed cores and rims and single plagioclase crystals from samples of the Skye Volcanic Centre, part of the British Palaeogene Igneous Province. The isotope analyses reveal that plagioclase crystals from different volcanic samples through thelava succession in Skye (Skye Main Lava Series, Preshal More Basalts and Big-Feldspar Lavas) have larger Sr and Pb isotopic ranges than the isotopic ranges found in hundreds of whole-rock analyses. The large Sr and Pb isotopic variation in plagioclase crystals in Skye is due to variable degrees of crustal contamination of the parental magmas during fractional crystallization and during the ascent of such magmas to the surface. The isotope variation of plagioclase crystals in the early lavas (Skye Main Lava Series) reflects assimilation-fractional crystallization processes dominated by the lower crust, whereas plagioclase crystals from later erupted magmas (Big-Feldspar Lavas and Preshal More basalts) show assimilation of upper crust superimposed on previous lower crustal contamination processes. The variability in Sr and Pb isotopes documented in different crystals within the same rocks shows that the crystals have been aggregated in magmas from different sites of storage and differentiation during the ascent of the magmas to the surface.

Isotopic evidence for open system processes within the Campanian Ignimbrite (Campi Flegrei–Italy) magma chamber

New Sr and Nd isotope data for whole rocks, glasses and minerals are combined to reconstruct the nature and origin of mixing end-members of the 200 km3 trachytic to phonolitic Campanian Ignimbrite (Campi Flegrei, Italy) magmatic system. The least-evolved magmatic end-member shows equilibrium between host glass and the majority of the phenocrysts and is less radiogenic in Sr and Nd than the most-evolved magma. On the contrary, only the Fe-rich pyroxene from the most-evolved erupted magma is in equilibrium with the matrix glass, while all other minerals are in isotopic disequilibrium. These magmas mixed prior to and during the Campanian Ignimbrite eruption and minerals were freely exchanged between the magma batches. Combining the results of the geochemical investigations on magma end-members with geophysical and geological data, we develop the following scenario. In stage 1, a parental, less differentiated magma rose into the middle crust, and evolved through combined crustal assimilation and crystal fractionation. In stage 2, the differentiated magma rose to shallower depth, fed the pre-Campanian Ignimbrite activity and evolved by further open-system processes into the most-evolved and most-radiogenic Campanian Ignimbrite end-member magma. In stage 3, new trachytic magma, isotopically distinct from the pre-Campanian Ignimbrite magmas, rose from ca. 6 km to shallower depth, recharged the most-evolved pre-Campanian Ignimbrite magma chamber, and formed the large and stratified Campanian Ignimbrite magmatic system. During the course of the Campanian Ignimbrite eruption, the two layers were tapped separately and/or simultaneously, and gave rise to the range of chemical and isotopic values displayed by the Campanian Ignimbrite pumices, glasses and minerals.

A cryptic record of magma mixing in diorites revealed by high-precision SIMS oxygen isotope analysis of zircons

High-precision in-situ ion microprobe (SIMS) oxygen isotope analysis of zircons from two diorite intrusions associated with the late Caledonian Lochnagar pluton in Scotland has revealed large differences in the degree of heterogeneity in zircon δ 18O between the diorites. Zircon crystals from the Cul nan Gad diorite (CnG) show a unimodal distribution of oxygen isotope values ( δ 18O = 6.0 ± 0.6‰ (2 σ)) and no or only minor grain-scale variation. Those from the Allt Darrarie diorite (AD1) show a large range in δ 18O and an apparent bimodal distribution with modes of 6.6 ± 0.4‰ and 7.3 ± 0.4‰. Variations of up to 1.2‰ occur between and within grains; both an increase and decrease in δ 18O with zircon growth has been observed. The δ 18O composition of growing zircon can only change if open-system processes affect the magma composition, i.e. if material of contrasting δ 18O composition is added to the magma. The variability in AD1 is interpreted to represent a cryptic record of magma mixing. A ‘deep crustal hot zone’ is a likely site for generation of the dioritic magmas which developed by mixing of residual melts and crustal partial melts or by melting of mafic lower crustal rocks. The overall small number of zircons with mantle-like δ 18O values (5.3 ± 0.6‰ (2 σ)) in the Lochnagar diorites is largely the product of crustal differentiation rather than crustal growth. The δ 18O of quartz from the CnG and AD1 diorites shows only minor variation (CnG: 10.9 ± 0.5‰ (2 σ), AD1: 11.7 ± 0.6‰ (2 σ)) within single populations, with no evidence of mixing. Quartz–zircon isotopic disequilibrium is consistent with later crystallisation of quartz from late magmatic fluids, and in case of the AD1 diorite after the inferred magma mixing from a homogenised, higher δ 18O melt. High-precision SIMS oxygen isotope analysis of zircon provides a new approach to identifying and resolving previously undetected early-stage magma mixing and constraining the compositions and origins of the component magmas. A combination of zircon, quartz and whole-rock data has proven to be a powerful tool in reconstructing the petrogenetic evolution of diorite from early crystallisation to late alteration.

Sources and evolution of arc magmas inferred from coupled O and Hf isotope systematics of plutonic zircons from the Cretaceous Separation Point Suite (New Zealand)

Coupled O and Hf isotopic compositions of zircons from Early Cretaceous (113–124 Ma) granitoids of the Separation Point Suite (SPS), New Zealand, obtained by cathodoluminescence imaging-guided micro-beam methods (SIMS, LA-ICPMS), are used as a record of evolving magma compositions in a prominent Mesozoic arc system. Eight representative SPS samples from individual plutons in the Nelson and Fiordland regions yield magmatic zircons with initial Hf isotope ratios (expressed in ε Hf) ranging from − 4 to + 11 (Nelson) and + 5 to + 12 (Fiordland), respectively. Initial Hf isotope ratios of zircons are extremely heterogeneous within individual samples, with the vast majority of values distinctly less radiogenic than depleted mantle at ~ 120 Ma ( ε Hf ~ + 16). O isotope ratios are likewise variable, with δ 18O zircon (SMOW) values of 2–8‰ (Nelson) and 0–7‰ (Fiordland). The within-rock variability in both zircon Hf and O isotope ratios is testimony to open-system processes that operated during magma evolution and zircon crystallisation. Average δ 18O zircon for cores and rims allow constraints to be placed on the O isotopic composition of magmas from which zircon precipitated ( δ 18O magma ~ 4–8‰). Elevated δ 18O magma (> 6.5‰) require involvement of 18O-enriched supracrustal material (weathered continental crust or low- T seawater-hydrothermally altered oceanic crust), while δ 18O magma < 5.5‰ imply contribution from a 18O-depleted crustal component. Whole rock Sr isotope, Nb/Ta and Nd/Pb systematics are inconsistent with 18O-depleted slab melts ( δ 18O ~ 0–6‰) as a source component for SPS magmas. Instead, low δ 18O values suggest incorporation of high- T meteoric-hydrothermally altered country rocks similar to those of the Largs terrane presently exposed in northern Fiordland. In diagrams of SiO 2 versus inferred δ 18O magma the most primitive SPS samples from the Nelson and Fiordland regions plot close to the expected composition for primitive arc magmas. More evolved granitoids, however, show strongly divergent trends of O isotope composition as a function of SiO 2, suggestive of assimilation coupled to fractional crystallisation (AFC) in contrasting crustal environments. Emplacement-level contamination by local crust is supported by age distributions of inherited zircons, which indicate a predominance of Palaeozoic and Mesozoic zircons in Nelson and Fiordland granitoids, respectively.

Mineral Textures and Zoning as Evidence for Open System Processes

Investigations of mineral textures and zoning as evidence for open system processes during magmatic evolution have always been a centerpiece of petrological studies and have provided some of the best evidence for magma mixing and crustal contamination for many decades (e.g., Milch 1905; Kuno 1936; Eichelberger 1975; Sato 1975; Anderson 1976). In fact, evidence from mineral studies was instrumental in the acceptance of magma mixing as important petrological process ever since it was initially proposed by Bunsen (1851). In recent years, mineral studies are invigorated by the development of high-precision, high-resolution analytical instruments and techniques through which textural information and compositional data are combined (e.g., Jerram and Davidson 2007). As minerals respond texturally and compositionally to changing magmatic environments, they preserve in their crystal growth stratigraphy a wealth of information regarding their past history of magmatic processes and compositions (cf. Ginibre et al. 2007). On the other hand, magmatic liquids (melts) are snapshots of current magmatic states, and provide less direct evidence of the processes responsible for their evolution. In addition, liquids may crystallize thereby destroying the direct evidence they provide. With an appropriate set of observations and measurements, we can correlate textures with mineral compositions and thus produce a richer composite picture of magmatic evolution than compositional data of minerals alone. This contribution reviews the zoning and textures of minerals frequently encountered in volcanic rocks (many of which apply equally to plutonic rocks) and the interpretations ascribed to these features in terms of open system magmatic processes. First, I will address the analytical tools that are used to acquire textural and compositional data, followed by a brief review of open system processes and a summary of the relevant textures. From that point on, I will address some mineral specific aspects followed by a discussion …

Deciphering Magma Chamber Dynamics from Styles of Compositional Zoning in Large Silicic Ash Flow Sheets

The understanding of the dynamic processes in magmatic systems has grown and changed markedly in the last decade. Old models for magmatic systems as vats of near-liquidus material have been revised by observations from seismology (Sinton and Detrick 1992), crystal chemistry and zoning (Davidson et al. 2007) and the geochronology and geochemistry of both plutonic and volcanic systems (Hildreth 2004; Charlier et al. 2007; Miller et al. 2007; Peressini et al. 2007; Walker et al. 2007). New views emphasise magmatic systems as temporally dominated by crystal mushes (magma bodies with a high fraction of solid particles; see definition in Miller and Wark 2008), that wax and wane in temperature and crystallinity, and are subject to significant open system processes (Charlier et al. 2007; Hildreth and Wilson 2007; Walker et al. 2007; Bachmann and Bergantz 2008). These processes, such as magma reintrusion, mixing, gas sparging, and subsequent thermal rejuvenation, may be significantly more important in producing the characteristics of a magmatic system than the previous closed-system, near-liquidus behaviour would predict. There are a number of recent reviews that summarize these observations (for example see Eichelberger et al. 2006; Bachmann et al. 2007b; Lipman 2007). Our aim here is to illustrate how this new perspective to magma dynamics is motivated by observations of heterogeneities (or lack thereof) in erupted rocks (and to a lesser amount in plutons). Owing to rapid withdrawal and quenching of magma during explosive volcanic eruptions (hours to a few days), large-volume (>1 km3) pyroclastic deposits (also referred to as ignimbrites or ash-flow tuffs ) provide an instant image of the state of the magma chamber evacuated during eruption. A first-order observation that characterizes these pyroclastic deposits of intermediate to silicic composition is that many do not tap into chemically …

Introduction to Minerals, Inclusions and Volcanic Processes

Minerals are intrinsically resistant to the processes that homogenize silicate liquids—their compositions thus yield an archive of volcanic and magmatic processes that are invisible at the whole rock scale. Minerals and their inclusions record diverse magma compositions, the depths and temperatures of magma storage, the nature of open system processes, and the rates at which magmas ascend. The potential for understanding volcanic systems through minerals and their inclusions has long been recognized (Sorby 1858). Sorby’s (1863) study of James Hall’s reversal experiments helped resolve the “basalt controversy” in favor of a volcanic origin, while Zirkel’s (1863) discovery of quartz within a volcanic rock helped tip the balance in favor of a magmatic origin for granite (Young 2003). Studies of phenocrysts have also long illustrated the importance of wall rock assimilation and magma mixing (e.g., Fenner 1926; Finch and Anderson 1930; Larson et al. 1938), and the potential for geothermometry (Barth 1934). Darwin’s (1844) mineralogical field-studies in the Galapagos archipelago, followed by King’s (1878) studies at Hawaii, also inaugurated the establishment of fractional crystallization as an important evolutionary process (Becker 1897; Bowen 1915). Recent advances in micro-analytical techniques open a new realm of detail, building upon a long history of mineralogical research; this volume summarizes some of this progress. Our summary focuses on volcanologic and magmatic processes, but the methods reviewed here extend well beyond terrestrial applications. Samples from the Stardust return mission, for example, show that olivine, plagioclase and pyroxene pervade the solar system (Brownlee et al. 2006)—while the topics covered here surely apply to all terrestrial-like planetary bodies, relevance may extend to a cosmic scale. Our more modest hope is that this volume will aid the study of disparate fields of terrestrial igneous systems, and perhaps provide a catalyst for new collaborations and integrated studies. …

High-precision U-series measurements of more than 500,000 year old fossil corals

Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ∼ 600 ka samples with an uncertainty of better than ± 15 ka (2 σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2 σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [Villemant B. and Feuillet N. (2003) Dating open systems by the 238U– 234U– 230Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman M.W., Goldstein S.L., and Speed R.C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of 230Th and 234U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model.