Open-system Magmatic Processes Research Articles

Multi-scale, open-system magmatic and sub-solidus processes contribute to the chemical and isotopic characteristics of the Jurassic Guadalupe Igneous Complex, Sierra Nevada, California, USA

Abstract The chemical and isotopic characteristics of a solidified pluton represent the integration of magmatic and sub-solidus processes operating across a range of spatial and temporal scales during pluton construction, crystallization, and cooling. Disentangling these processes and understanding where chemical and isotopic signatures were acquired requires the combination of multiple tools tracing processes at different time and length scales. We combine whole-rock oxygen and Sr-Nd isotopes, zircon oxygen isotopes and trace elements, and mineral compositions with published high-precision U-Pb zircon geochronology to evaluate differentiation within the bimodal Guadalupe Igneous Complex, Sierra Nevada, California (USA). The complex was constructed in ~300 k.y. between 149 and 150 Ma. Felsic magmas crystallized as centimeter- to meter-sized segregations in gabbros in the lower part of the complex and as granites and granophyres structurally above the gabbros. A central mingling zone separates the mafic and felsic units. Pluton-wide δ18O(whole-rock), δ18O(zircon), and Sr-Nd isotopic ranges are too large to be explained by in situ, closed-system differentiation, instead requiring open-system behavior at all scales. Low δ18O(whole-rock) and δ18O(zircon) values indicate assimilation of hydrothermally altered marine host rocks during ascent and/or emplacement. In situ differentiation processes operated on a smaller scale (meters to tens of meters) for at least ~200 k.y. via (1) percolation and segregation of chemically and isotopically diverse silicic interstitial melt from a heterogeneous gabbro mush; (2) crystal accumulation; and (3) sub-solidus, high-temperature, hydrothermal alteration at the shallow roof of the complex to modify the chemical and isotopic characteristics. Whole-rock and mineral chemistry in combination with geochronology allows deciphering open-system differentiation processes at the outcrop to pluton scale from magmatic to sub-solidus temperatures over time scales of hundreds of thousands to millions of years.

Bulk-rock and mineral-scale geochemical evidence of open-system magmatic processes in the felsic rocks of the northern part of the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

Bulk-rock and mineral-scale geochemical evidence of open-system magmatic processes in the felsic rocks of the northern part of the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

Multi-scale isotopic heterogeneity reveals a complex magmatic evolution: An example from the wallundry suite granitoids of the lachlan fold belt, Australia

Open-system magmatic processes are expected to impart various sorts of isotopic heterogeneity upon the igneous rocks they produce. The range of processes under the “open-system” umbrella (e.g., simple two-component mixing, magma mingling, assimilation with fractional crystallization) cannot usually be uniquely identified using data from a single isotope system. The use of bulk-rock, mineral separate and in situ techniques and multiple isotope systems allows the characterization of isotopic variability at different sampling scales, illuminating details of the petrogenesis of a magmatic system. This approach has been applied to granitoids of the Wallundry Suite in the Lachlan Fold Belt, Australia. The Wallundry Suite exhibits variations in mineral assemblage, mineral composition and trends in bulk-rock major and trace element compositions consistent with the involvement of liquid-crystal sorting processes such as fractional crystallization. In situ paired O-Hf isotope data from zircon in six samples show an array indicating the isotopic evolution of the melt phase. Similarly, bulk-rock Sr-Nd-Hf isotope arrays support open-system magma evolution. These data combined with the petrographic observations and major and trace element geochemical variations suggest some form of assimilation-fractional crystallization process in the petrogenesis of the Wallundry Suite. Added complexity is revealed by two observations: 1) the isotopic variations are only weakly coupled to the lithology and major element compositions of the samples; and 2) there are distinguishable differences between the Hf isotope compositions of bulk-rock samples and those of the magmatic zircons they host. To varying degrees the rocks consistently show negative ΔεHfbulk-zrc values (i.e., the bulk-rock compositions have less radiogenic Hf isotope values than their coexisting zircons). The preservation of distinctly low Nd and Hf isotope ratios in an Fe-Ti oxide mineral separate suggests that the bulk-rock vs. zircon discrepancy is caused by the presence of unmelted components derived from a contaminant of continental origin (i.e., a rock with low Sm/Nd and Lu/Hf and thus unradiogenic Nd and Hf). Evidently, a complex interplay of assimilation, crystallization and melt segregation is required to account for the data. This investigation demonstrates that such complexity can, nevertheless, be disentangled through comparison of complementary isotope data at multiple sampling scales.

Magmatic crystallization drives zircon Zr isotopic variations in a large granite batholith

Stable zirconium (Zr) isotope of magmatic zircons is a promising new tracer to understand magma differentiation in the continental crust. However, magmatic processes controlling zircon Zr isotopic variations remain poorly constrained. Here we present high-spatial-resolution in-situ methods on magmatic zircons for U-Pb age, trace elements, Hf isotopes, and stable Zr isotopes (δ94/90Zr relative to IPGP-Zr) as well as bulk rock δ94/90Zr for a large granite batholith (SiO2 = ∼73 wt%) in the Jiaodong Peninsula, eastern North China Craton. Magmatic zircons are classified into high-luminance Type-I and low-luminance Type-II zircons in the cathodoluminescence images. Both types show indistinguishable U-Pb ages and initial Hf isotopic compositions, indicating their same magma source. Yet, they differ in chemical compositions and δ94/90Zr values. Type-I zircons display lower δ94/90Zr values (−0.35‰ to 0.15‰) than Type-II zircons (0.16–0.54‰). The δ94/90Zr values overall increase with decreasing Zr/Hf, Th/U ratios, Ti-in-zircon temperature, and increasing U abundances in zircons, implying that zircon δ94/90Zr values becomes heavier with the enhanced magma differentiation. Such concomitant correlations resulted from closed-system magma crystallization without noticeable segregation of feldspar and zircon, as revealed by high Sr and Ba content, limited Eu anomalies, and mantle-like δ94/90Zr in the bulk rocks (−0.01 ± 0.07‰ and 0.02 ± 0.04‰). Thus, Type-I zircons incorporating light Zr isotopes crystallized at an early stage of magma solidification, while Type-II zircons grew from more evolved residual melts with elevated δ94/90Zr values. Zoned zircon grains with Type-I cores and Type-II rims record the prolonged crystallization history covering both stages with contrasting melt compositions; whereas oscillation zircon grains of Type-I or Type-II, crystallizing at the stable melt composition, display the restricted intragrain variations of δ94/90Zr, Zr/Hf, and Th/U ratios. These results demonstrate that closed-system magmatic crystallization plays a critical role in zircon Zr isotopic variations. We further propose that the variability of δ94/90Zr values and multiple types of Zr isotopic profiles in zircons are predominantly driven by the compositional effect of adjacent melts from which zircons crystallize. The composition-related δ94/90Zr variations provide a fundamental framework for understanding Zr isotopic evolution in the silicic igneous system and can be used for exploring open-system magmatic processes in the mushy pluton bodies, which are important for continental crust evolution.

Petrographic evidences of open-system magmatic processes in the felsic rocks of the northern part of the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

Abstract Over the almost 190 years-long research of the Ditrău Alkaline Massif (Eastern Carpathians, Romania), felsic rocks have been regarded as homogeneous, uniform units of the igneous complex. Nevertheless, our detailed textural study revealed that the felsic suite (diorite–alkaline feldspar syenite and nepheline-bearing syenite–granite series) cropping out north of the Jolotca Creek valley is more heterogenous at micro-scale than previously thought. This heterogeneity partly derives from abundant mafic mineral-rich clusters; nevertheless, felsic minerals also exhibit various, remarkable textural features. Outcrop to micro-scale traits of felsic crystal settling, mafic mineral aggregates and flow fabrics along with metamorphic country rock xenoliths suggest that the studied rocks crystallized under dynamic magmatic conditions. Cumulate formation, shear flow, convection currents as well as various open-system magmatic processes (e.g., magma recharge, magma mixing and mingling, crystal or mush transfer and recycling, country rock assimilation) played a significant role in the petrogenesis of the examined felsic suite. Based on field observations as well as on the microtextural relationship of the minerals, two major groups of felsic rocks were distinguished: (1) felsic rocks (lacking or containing sparse mafic minerals) spatially associated with mafic rocks and (2) felsic rocks (with mafic minerals and clots) spatially unassociated with mafic rocks. Rocks of the former group are dominated by plagioclase, accompanied by minor alkaline feldspar, biotite and accessory titanite. Distinct structural and textural features suggest the physical accumulation of the rock-forming phases. Such textural properties can also be observed in some rocks of the second group. Isolated mafic minerals are rather scarce in the latter; nevertheless, different types of aggregates made up of either identical or various mafic phases are more common. Clustered minerals are either intact or show different stages of alteration. A detailed petrographic study of the above-mentioned peculiarities has been implemented in order to define their potential origin(s) and petrogenetic significance.

Mineral-Melt Equilibria and Geothermobarometry of Campi Flegrei Magmas: Inferences for Magma Storage Conditions

The eruptions of Campi Flegrei (Southern Italy), one of the most studied and dangerous active volcanic areas of the world, are fed by mildly potassic alkaline magmas, from shoshonite to trachyte and phonotrachyte. Petrological investigations carried out in past decades on Campi Flegrei rocks provide crucial information for understanding differentiation processes in its magmatic system. However, the compositional features of rocks are a palimpsest of many processes acting over timescales of 100–104 years, including crystal entrapment from multiple reservoirs with different magmatic histories. In this work, olivine, clinopyroxene and feldspar crystals from volcanic rocks related to the entire period of Campi Flegrei’s volcanic activity are checked for equilibrium with combined and possibly more rigorous tests than those commonly used in previous works (e.g., Fe–Mg exchange between either olivine or clinopyroxene and melt), with the aim of obtaining more robust geothermobarometric estimations for the magmas these products represent. We applied several combinations of equilibrium tests and geothermometric and geobarometric methods to a suite of rocks and related minerals spanning the period from ~59 ka to 1538 A.D. and compared the obtained results with the inferred magma storage conditions estimated in previous works through different methods. This mineral-chemistry investigation suggests that two prevalent sets of T–P (temperature–pressure) conditions, here referred to as “magmatic environments”, characterized the magma storage over the entire period of Campi Flegrei activity investigated here. These magmatic environments are ascribable to either mafic or differentiated magmas, stationing in deep and shallow reservoirs, respectively, which interacted frequently, mostly during the last 12 ka of activity. In fact, open-system magmatic processes (mixing/mingling, crustal contamination, CO2 flushing) hypothesized to have occurred before several Campi Flegrei eruptions could have removed earlier-grown crystals from their equilibrium melts. Moreover, our new results indicate that, in the case of complex systems such as Campi Flegrei’s, in which different pre-eruptive processes can modify the equilibrium composition of the crystals, one single geothermobarometric method offers little chance to constrain the magma storage conditions. Conversely, combined methods yield more robust results in agreement with estimates obtained in previous independent studies based on both petrological and geophysical methods.

Decoupled whole-rock and zircon Hf isotopes in young evolved post-collisional lavas from Dayingshan (SE Tibet): Evidence for open-system magmatic processes

Young zircons from volcanic rocks provide ideal samples for evaluating whole-rock and zircon Hf isotope equilibrium. We report zircon and whole-rock Hf isotopic compositions for Holocene post-collisional lavas (trachyandesites and trachytes) from Dayingshan volcano, SE Tibetan Plateau. Individual zircon εHf values differ from the host whole-rock value by up to 7.4ε units, and average zircon εHf values in individual samples are about 3ε units lower than the corresponding whole-rock εHf values, indicating discrepancy of zircon and whole-rock Hf isotopes. δ18O values for Dayingshan zircons vary from 6.1 to 7.8‰ and are negatively correlated with zircon εHf values. The decoupling of zircon/whole-rock Hf isotopic compositions is attributed to open-system magmatic processes rather than disequilibrium partial melting, as indicated by the negative correlation of zircon Hf-O isotopic compositions, as well as the correlations between whole-rock SiO2 content and Nd-Sr-Hf isotope ratios. The decoupling of zircon/whole-rock Hf isotopic compositions in Dayingshan lavas suggest that care must be taken when using individual zircon εHf values to study crust-mantle evolution. This Hf isotope decoupling might be more common than previously thought in continental post-collisional volcanic rocks where open-system magmatic processes are common.

A Pilbara perspective on the generation of Archaean continental crust

Models for the generation and evolution of continental crust are informed by a detailed understanding of regional geology and geochemistry. This contribution therefore evaluates the increasing amounts of high quality geochemical data, on well-characterized samples, now available from the magmatic rocks of the Pilbara Craton, Western Australia. The East Pilbara Terrane is characterized by vertical dome and basin tectonics, and the younger (Mesoarchean) rocks to the west are associated with lateral tectonics. The craton stabilized at ~2.95 Ga (Hickman, 2012), and it was overlain by voluminous, dominantly mafic lavas of the Fortescue Group in an intra-cratonic setting at ~2.7 Ga. The magmatic rocks of the Pilbara Craton are bimodal in SiO2, with the volcanic rocks typically having <60% SiO2, and the plutonic rocks >60% SiO2, and such differences are in part linked to different depths of magma emplacement.The volcanic rocks are divided into two groups, with Dy/La > and < 0.6. The geochemistry of the former group is consistent with broadly closed system differentiation, whereas the low Dy/La rocks largely reflect open system magmatic processes, and contributions from high silica magmas. The high Dy/La rocks have restricted Th/Nb ratios similar to those of primitive mantle, whereas the low Dy/La rocks range to higher Th/Nb values. In most cases these higher Th/Nb values reflect open system magma differentiation, but in some of the younger volcanic suites (the Bookingarra Group and Cooneeina Basalt) these values appear to be associated with subduction. There is no evidence for garnet or amphibole in the generation of the volcanic rocks, however the composition of the plutonic rocks reflects the involvement of both garnet and plagioclase, during melt generation and differentiation.The high Dy/La rocks have Sm/Nd ratios similar to CHUR, but their Lu/Hf ratios are significantly sub-chondritic. Many of the Pilbara rocks from 3.5–3.0 Ga were generated from mantle sources with broadly chondritic Hf isotope ratios, and so in the preferred model the mean composition of the high Dy/La group was generated by ~20% melting of chondritic mantle. Apart from the high Th/Nb magmas at 3.05–2.97 Ga (Smithies et al., 2005), there are surprisingly few changes in the minor and trace element features of the magmas associated with the generation of the craton (3.5–3.0 Ga), and after it stabilized at ~2.95 Ga. The 3.05–2.97 Ga plutonic rocks in the granitic complexes also have elevated Th/Nb, suggesting that such ratios were inherited from their mafic source rocks. This observation implies that the mafic sources and the granitic rocks were of similar age, and that there was rapid conversion of young mantle-derived basaltic crust into felsic continental crust, with little contribution from older basement material. The volcanic rocks are characterized by relatively low water contents (~<2%) on the basis of their low maximum Al2O3 values, a mean Fe3+/ΣFe of 0.19 ± 0.01, and models in which the high Dy/La rocks were derived from primitive mantle. Such low water contents relative to modern subduction-related magmas are a feature of the magmatic rocks of the Pilbara Craton, and the development of ‘granitic crust’ appears to be linked to the development of higher water contents associated with subduction, locally commencing at ca. 3.1 Ga, and to globally dominant plate tectonics.

Open-system magmatic evolution and crystallization conditions of the Ediacaran shoshonitic rocks from the São João do Sabugi Pluton, Borborema Province, NE Brazil

Abstract The Sao Joao do Sabugi Pluton (SJSP) (∼579 Ma) is one of the most expressive occurrences of Ediacaran, syn- to post-collisional shoshonitic rocks in the Rio Piranhas-Serido Domain, Borborema Province, NE Brazil. The petrography, whole rock and mineral composition of this pluton were investigated to characterize its open-system magmatic differentiation processes. The pluton is composed of three rock groups: gabbro-diorite, with clinopyroxene ± orthopyroxene ± amphibole ± biotite; monzodiorite, with amphibole and biotite (± pyroxenes); and granodiorite, with biotite as the main mafic mineral. Evidences of open-system processes, including partially resorbed metasediment xenoliths, are widespread. These rocks are metaluminous, alkali-calcic and magnesian, and have shoshonitic affinity. They show relative LILE and LREE enrichment and pronounced negative Nb-Ta anomalies. Major and trace element geochemical modeling favors magma differentiation by fractional crystallization (56–62%) after crustal assimilation of ∼30% of local paragneisses and schists from the Serido Group in a relatively stationary magma chamber at crustal depth, under pressures of 5–6 kbar, temperatures of ∼800–1,000°C and relatively oxidizing conditions. The less evolved gabbro-diorite was generated by ∼18% modal dynamic partial melting of a metasomatized mantle source.

Diagnosing open-system magmatic processes using the Magma Chamber Simulator (MCS): part I—major elements and phase equilibria

The Magma Chamber Simulator (MCS) is a thermodynamic tool for modeling the evolution of magmatic systems that are open with respect to assimilation of partial melts or stoped blocks, magma recharge + mixing, and fractional crystallization. MCS is available for both PC and Mac. In the MCS, the thermal, mass, and compositional evolution of a multicomponent–multiphase composite system of resident magma, wallrock, and recharge reservoirs is tracked by rigorous self-consistent thermodynamic modeling. A Recharge–Assimilation (Assimilated partial melt or Stoped blocks)–Fractional Crystallization (RnASnFC; ntot ≤ 30) scenario is computed by minimization or maximization of appropriate thermodynamic potentials using the family of rhyolite- and pMELTS engines coupled to an Excel Visual Basic interface. In MCS, during isobaric cooling and crystallization, resident magma thermally interacts with wallrock that is in internal thermodynamic equilibrium. Wallrock partial melt above a user-defined percolation threshold is homogenized (i.e., brought in to chemical potential equilibrium) with resident magma. Crystals that form become part of a cumulate reservoir that remains thermally connected but chemically isolated from resident melt. Up to 30 instances (n ≤ 30) of magma mixing by recharge and/or bulk assimilation of stoped wallrock blocks can occur in a single simulation; each recharge magma or stoped block has a unique user-defined composition and thermal state. Recharge magmas and stoped blocks hybridize (equilibrate) with resident melt, yielding a single new melt composition and temperature. MCS output includes major and trace element concentrations and isotopic ratios (Sr, Nd, Hf, Pb, Os, and O as defaults) of wallrock, recharge magma/stoped blocks, resident magma melt, and cumulates. The chemical formulae of equilibrium crystalline phases in the cumulate reservoir, wallrock, and recharge magmas/stoped blocks are also output. Depending on the selected rhyolite-MELTS engine, the composition and properties of a possible supercritical fluid phase (H2O and/or CO2) are also tracked. Forward modeling of theoretical magma systems and suites of igneous rocks provides quantitative insight into key questions in igneous petrology such as mantle versus crustal contributions to terrestrial magmas, the record of magmatism preserved in cumulates and exsolved fluids, and the chronology of RASFC processes that may be recorded by crystal populations, melt inclusions, and whole rocks. Here, we describe the design of the MCS software that focuses on major element compositions and phase equilibria (MCS-PhaseEQ). Case studies that involve fractional crystallization, magma recharge + mixing, and crustal contamination of a depleted basalt that resides in average upper crust illustrate the major element and phase equilibria consequences of these processes and highlight the rich array of data produced by MCS. The cases presented here, which represent an infinitesimal fraction of possible RASFC processes and bulk compositions, show that the records of recharge and/or crustal contamination may be subtle and are not necessarily those that would be predicted using conventional intuition and simple mass balance arguments. Mass and energy constrained thermodynamic tools like the MCS quantify the open-system evolution of magmas and provide a systematic understanding of the petrology and geochemistry of open system magmatic processes. The trace element and isotope MCS computational tool (MCS-Traces) is described in a separate contribution (part II).

Diagnosing open-system magmatic processes using the Magma Chamber Simulator (MCS): part II\u2014trace elements and isotopes

The Magma Chamber Simulator (MCS) is a thermodynamic model that computes the phase, thermal, and compositional evolution of a multiphase–multicomponent system of a Fractionally Crystallizing resident body of magma (i.e., melt ± solids ± fluid), linked wallrock that may either be assimilated as Anatectic melts or wholesale as Stoped blocks, and multiple Recharge reservoirs (RnASnFC system, where n is the number of user-selected recharge events). MCS calculations occur in two stages; the first utilizes mass and energy balance to produce thermodynamically constrained major element and phase equilibria information for an RnASnFC system; this tool is informally called MCS-PhaseEQ, and is described in a companion paper (Bohrson et al. 2020). The second stage of modeling, called MCS-Traces, calculates the RASFC evolution of up to 48 trace elements and seven radiogenic and one stable isotopic system (Sr, Nd, Hf, 3xPb, Os, and O) for the resident melt. In addition, trace element concentrations are calculated for bulk residual wallrock and each solid (± fluid) phase in the cumulate reservoir and residual wallrock. Input consists of (1) initial trace element concentrations and isotope ratios for the parental melt, wallrock, and recharge magmas/stoped wallrock blocks and (2) solid-melt and solid–fluid partition coefficients (optional temperature-dependence) for stable phases in the resident magma and residual wallrock. Output can be easily read and processed from tabulated worksheets. We provide trace element and isotopic results for the same example cases (FC, R2FC, AFC, S2FC, and R2AFC) presented in the companion paper. These simulations show that recharge processes can be difficult to recognize based on trace element data alone unless there is an independent reference frame of successive recharge events or if serial recharge magmas are sufficiently distinct in composition relative to the parental magma or magmas on the fractionation trend. In contrast, assimilation of wallrock is likely to have a notable effect on incompatible trace element and isotopic compositions of the contaminated resident melt. The magnitude of these effects depends on several factors incorporated into both stages of MCS calculations (e.g., phase equilibria, trace element partitioning, style of assimilation, and geochemistry of the starting materials). Significantly, the effects of assimilation can be counterintuitive and very different from simple scenarios (e.g., bulk mixing of magma and wallrock) that do not take account phase equilibria. Considerable caution should be practiced in ruling out potential assimilation scenarios in natural systems based upon simple geochemical “rules of thumb”. The lack of simplistic responses to open-system processes underscores the need for thermodynamical RASFC models that take into account mass and energy conservation. MCS-Traces provides an unprecedented and detailed framework for utilizing thermodynamic constraints and element partitioning to document trace element and isotopic evolution of igneous systems. Continued development of the Magma Chamber Simulator will focus on easier accessibility and additional capabilities that will allow the tool to better reproduce the documented natural complexities of open-system magmatic processes.

Petrology and geochemistry of early Carboniferous volcanic rocks in the Xinyuan region of western Tianshan: Implications for magma sources

Understanding the sources and processes that produce fossil continental arc magmas is still a challenging problem. To address this problem, here we perform detailed petrological investigations as well as whole-rock geochemical and SrNd isotopic analyses on the early Carboniferous volcanic rocks in the Xinyuan region. These volcanic rocks mainly consist of calc-alkaline basalt, trachy-andesite, and rhyolite, and show subduction-related geochemical and isotopic signatures. Petrographic observations and new geochemical data suggest that the rhyolite was generated by partial melting of juvenile crustal materials, while the basalt and trachy-andesite might undergo a series of open-system magmatic processes, such as fractional crystallization, crustal contamination, and magma replenishment and mixing. The primitive basaltic rocks with Mg# values of ~70 were least affected by crustal-level magmatic processes, and they could be used to constrain sub-arc mantle processes. The Nb/Yb, Zr/Yb, and Ta/Yb ratios of these basaltic rocks are similar to those of E-MORB, suggesting a relatively fertile mantle source prior to subduction, while their high Ba/Th, Sr/Nd, and Th/Nd ratios suggest a significant contribution of slab-derived components to the pre-subduction mantle wedge. Geochemical modelling based on trace elements and SrNd isotopic ratios indicates that the pre-subduction mantle wedge was modified by ~0.5% subducted sediment-derived fluids and ~2% bulk sediments. About 10% partial melting of such enriched mantle source could produce the early Carboniferous primitive magmas in the Xinyuan region.

Chemical mass balance equations for open-system magma chamber processes that result in crystal zoning

Recent advances in micro-analysis have revealed that isotopic zoning profiles in igneous crystals reflect progressive changes in liquid composition during open-system magmatic processes. Consequently, the isotope ratio of the integrated (bulk) crystals and that of liquid are inevitably different. This study develops a mass balance model to describe the compositional relationships between the bulk crystals (total suspended crystals), liquid, magma (crystals plus liquid), and crystal rims during open-system magma chamber processes. The model incorporates the effects of concurrent magma recharge (or mixing), assimilation, magma extraction, and fractional crystallization. The mass balance differential equations for suspended crystals and liquid are solved simultaneously. The analytical solution of the equations gives a quantitative account of the evolution paths of trace elements and isotopes within total suspended crystals, liquid, magma, and crystal rims. The isotopic zoning profiles in phenocrysts vary markedly depending on the relative rates of recharge, assimilation, magma extraction, and fractional crystallization. The model can reproduce the 87Sr/86Sr zoning patterns in natural feldspars from different magma systems.

Incremental Growth of Mid- to Upper-Crustal Magma Bodies During Arabia–Eurasia Convergence and Collision: A Petrological Study of the Calc-Alkaline to Shoshonitic Meghri–Ordubad Pluton (Southern Armenia and Nakhitchevan, Lesser Caucasus)

The composite Meghri-Ordubad pluton (MOP) is located in the southernmost Lesser Caucasus and outcrops over ~1000 km2 in southern Armenia and Nakhitchevan. It is characterized by nested intrusions incrementally emplaced over ~30 Myr from the Middle Eocene to Early Miocene, coeval with the closure of the Neotethyan Ocean and the regional Arabia-Eurasia continental collision. We recognize three compositionally distinct intrusive series in the MOP: a medium- to high-K calc-alkaline series 1 dominated by gabbro, quartz diorite and tonalite, a shoshonitic to high-K calc-alkaline series 2 including gabbro, monzogabbro, monzodiorite, monzonite and syenite, and a high-K calc-alkaline adakitic series 3 characterized by the emplacement of lamprophyre dikes followed by porphyritic granodioritic intrusions and dikes. Thermobarometry calculations together with detailed petrography, mineral chemistry and experimental mineral stability P-T diagrams indicate a polybaric crystallization history and magma emplacement at mid- to upper crustal levels (0.1-0.3 GPa). Mineral textures and compositions reveal open-system magmatic processes such as mafic magma replenishment and reactivation of crystal mushes prior to transport towards shallower crustal levels in the MOP. The evolution of magma chemistry (calc-alkaline vs shoshonitic) and mineral assemblages (amphibole-bearing vs amphibole-free) indicate fluctuation in primary melt alkali and water contents over time. Such chemical evolution is ascribed to a decrease in the degree of partial melting of a chemically heterogeneous, but isotopically homogeneous, mantle source. Overall, the MOP is characterized by the successive emplacement of hydrous magma batches in the mid- to upper crust and petrological processes that were dominated by fractional crystallization (± crustal assimilation) upon cooling and, or, decompression (± degassing). We propose a new temporal and spatial petrogenetic model for the MOP, encompassing three long-lived, chemically distinct magmatic series incrementally assembled over 30 Myr within a pre-, syn- and post-collisional geodynamic setting.

Zircon trace element and isotopic (Sr, Nd, Hf, Pb) effects of assimilation-fractional crystallization of pegmatite magma: A case study of the Guangshigou biotite pegmatites from the North Qinling Orogen, central China

Evidence for open-system magmatic processes related to wallrock assimilation accompanied by fractional crystallization (AFC) is present in the Guangshigou biotite pegmatites, North Qinling Orogen. The biotite pegmatite-gneiss contacts generally coincide with the greatest enrichment of U and Th. Zircon UPb dating constrains the crystallization ages of the biotite pegmatite (rim zone-415±2.6Ma; internal zone-413.5±2.5Ma), in line with a pyrite PbPb isochron age (413±22Ma). Metamict areas in zircon show generally elevated concentrations of trace elements and expulsion of radioactive Pb. Internal zone samples, representing uncontaminated magma, have negative to positive zircon (~413Ma) εHf(t) (−1.53−+3.24), low εNd(t) values (−2.4), and old Hf and Nd model ages (tDM2=1.5–1.19Ga, T2DM=1.35Ga, respectively), indicating a dominantly recycled Mesoproterozoic lower crustal material with involvement of some juvenile materials in the source region. The magmatic oxygen fugacity (fO2) and crystallization temperatures ranges from −24.81 to −13.34 of log fO2 and 570°C to 793°C, respectively. Compared to the internal zone, pegmatite rim samples display a variable and lower εNd(t) values (−3.9 to −2.8) and T2DM (1.47–1.37Ga), but similar Hf isotopic compositions, favouring a three-component isotopic mixing model (recycled Mesoproterozoic lower crust materials, juvenile materials, and host gneiss). Pronounced variations of Ti, Y, U, Th, Hf, and REE concentrations in zircon from grain to grain in individual samples and from area to area within individual grains suggest a fluctuating crystallization environment in hybridized magma from which the rim-hosted zircons crystallized. Variable and high radiogenic Pb ratios of pyrites forming in the hybridized magma were inherited from the matrix. Zircons from both zones exhibit similar Hf isotope patterns, indicating the rim-hosted zircons crystallized during the early stage of hybridization of magma. However, the heterogeneous Nd isotopic composition in individual pegmatites imply an incorporation of wallrock-derived melt with less radiogenic Nd.

Sulfur isotopic zoning in apatite crystals: A new record of dynamic sulfur behavior in magmas

The mobility and geochemical behavior of sulfur in magmas is complex due to its multi-phase (solid, immiscible liquid, gaseous, dissolved ions) and multi-valent (from S2− to S6+) nature. Sulfur behavior is closely linked with the evolution of oxygen fugacity (fO2) in magmas; the record of fO2 evolution is often enigmatic to extract from rock records, particularly for intrusive systems. We apply a novel method of measuring S isotopic ratios in zoned apatite crystals that we interpret as a record of open-system magmatic processes. We interrogate the S concentration and isotopic variations preserved in multiple apatite crystals from single hand specimens from the Cadiz Valley Batholith, CA via electron microprobe and ion microprobe. Isotopic variations in single apatite crystals ranged from 0 to 3.8‰ δ34S and total variation within a single hand sample was 6.1‰ δ34S. High S concentration cores yielded high isotopic ratios while low S concentration rims yielded low isotopic ratios. We discuss a range of possible natural scenarios and favor an explanation of a combination of magma mixing and open-system, ascent-driven degassing under moderately reduced conditions: fO2 at or below NNO+1, although the synchronous crystallization of apatite and anhydrite is also a viable scenario. Our conclusions have implications for the coupled S and fO2 evolution of granitic plutons and suggest that in-situ apatite S isotopic measurements could be a powerful new tool for evaluating redox and S systematics in magmatic systems.

Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust

Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F–Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800–850 °C, >15 kbar). Zircons show four main age groupings: 1.0–0.5 Ga, 50–45, 35–20, and 16–13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50–45 Ma zircons match the crystallization age and juvenile character (eHf i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35–20 Ma Miocene ages are derived from zircons with similar Hf–O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon δ18O (+8.73–8.97 ‰) values, and extremely low eHf i (−12.7 to −9.4) values. Such evolved Hf–O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low δ18O (+6 to 7.5 ‰), intermediate (δ18O +8.5 to 9.0 ‰), and high δ18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ18O 10–12 ‰. Both the low and intermediate δ18O groups show significant variation in Fe content, whereas the two high δ18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35–20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust–mantle hybridization in the deep Tibetan crust.

Resolving discordant U–Th–Ra ages: constraints on petrogenetic processes of recent effusive eruptions at Tatun Volcano Group, northern Taiwan

Abstract U–Th–Ra isotope analyses of whole rocks and mineral separates were conducted in order to perform isochron dating of three morphologically young lavas from Tatun volcano, northern Taiwan (from Mt Cising, the Shamao dome and the Huangzuei volcano). The data do not yield tight U–Th isochrons, indicating open-system magmatic processes. However, crystallization ages of two samples can be constrained: namely, less than about 1370 years for the Shamao dome, based on 226 Ra– 230 Th disequilibrium in magnetite, and less than approximately 70 ka (but potentially Holocene) for a Huangzuei flow, based on 238 U– 230 Th disequilibrium in plagioclase. Discordant Ar–Ar, 238 U– 230 Th and 226 Ra– 230 Th ages are best explained by young lavas having inherited some crystals from older lithologies (crystal mushes or rocks), and indicate that the above ages represent maxima. Our study provides the first evidence of effusive volcanism at the Tatun Volcano Group in Late Holocene times. All separates from the Shamao dome and Huangzuei volcano are in 234 U– 238 U equilibrium. Minerals in the Mt Cising sample are in 234 U– 238 U disequilibrium, despite the 234 U– 238 U equilibrium of the whole rock. We interpret this as uptake of a hydrothermally altered, old crystal cargo into fresh melt prior to eruption. A different dating approach will thus be required to constrain the eruption age of Mt Cising. Supplementary material: Ar–Ar plateaus from Mt Cising and the Shamao dome, reproduced from Lee (1996), are available at www.geolsoc.org.uk/SUP18817

Apatite: A new redox proxy for silicic magmas?

The oxidation states of magmas provide valuable information about the release and speciation of volatile elements during volcanic eruptions, metallogenesis, source rock compositions, open system magmatic processes, tectonic settings and potentially titanium (Ti) activity in chemical systems used for Ti-dependent geothermometers and geobarometers. In this paper we explore the use of Mn in apatite as an oxybarometer in intermediate and silicic igneous rocks. Increased Mn concentrations in apatite in granitic rocks from the zoned Criffell granitic pluton (southern Scotland) correlate with decreasing Fe2O3 (Fe3+) and Mn in the whole-rock and likely reflect increased Mn2+/Mn3+ and greater compatibility of Mn2+ relative to Mn3+ in apatite under reduced conditions. Fe3+/Fe2+ ratios in biotites have previously been used to calculate oxygen fugacities (fO2) in the outer zone granodiorites and inner zone granites where redox conditions have been shown to change from close to the magnetite-hematite buffer to close to the nickel-nickel oxide buffer respectively (Stephens et al., 1985). This trend is apparent in apatite Mn concentrations from a range of intermediate to silicic volcanic rocks that exhibit varying redox states and are shown to vary linearly and negatively with log fO2, such that logfO2=-0.0022(±0.0003)Mn(ppm)-9.75(±0.46)Variations in the Mn concentration of apatites appear to be largely independent of differences in the Mn concentration of the melt. Apatite Mn concentrations may therefore provide an independent oxybarometer that is amenable to experimental calibration, with major relevance to studies on detrital mineral suites, particularly those containing a record of early Earth redox conditions, and on the climatic impact of historic volcanic eruptions.

Petrological insights into the storage conditions, and magmatic processes that yielded the centennial 2010 Merapi explosive eruption

Petrological insights into the storage conditions, and magmatic processes that yielded the centennial 2010 Merapi explosive eruption