Open-system Processes Research Articles

Quantum thermodynamics in adiabatic open systems and its trapped-ion experimental realization

Quantum thermodynamics aims at investigating both the emergence and the limits of the laws of thermodynamics from a quantum mechanical microscopic approach. In this scenario, thermodynamic processes with no heat exchange, namely, adiabatic transformations, can be implemented through quantum evolutions in closed systems, even though the notion of a closed system is always an idealization and approximation. Here, we begin by theoretically discussing thermodynamic adiabatic processes in open quantum systems, which evolve non-unitarily under decoherence due to its interaction with its surrounding environment. From a general approach for adiabatic non-unitary evolution, we establish heat and work in terms of the underlying Liouville superoperator governing the quantum dynamics. As a consequence, we derive the conditions that an adiabatic open-system quantum dynamics implies in the absence of heat exchange, providing a connection between quantum and thermal adiabaticity. Moreover, we determine families of decohering systems exhibiting the same maximal heat exchange, which imply in classes of thermodynamic adiabaticity in open systems. We then approach the problem experimentally using a hyperfine energy-level quantum bit of an Ytterbium 171Yb+ trapped ion, which provides a work substance for thermodynamic processes, allowing for the analysis of heat and internal energy throughout a controllable engineered dynamics.

Tracking the magmatic response to subduction initiation in the forearc mantle wedge: Insights from peridotite geochemistry of the Guleman and Kızıldağ ophiolites, Southeastern Turkey

The initiation of subduction is associated with sequential magmatic responses that lead to the formation of the forearc lithosphere, yet the detailed characteristics of these magmatic activities are not well constrained. Here we use mineral chemistries and bulk-rock trace-element contents of highly-depleted harzburgites from the Guleman and Kızıldağ ophiolites in Southeast Turkey to examine mantle wedge melting dynamics during subduction initiation. We focus on how different components from the subducting slabs potentially contribute to various stages of magmatism throughout the process. Mineral and bulk-rock compositions of these harzburgites are significantly different from those of abyssal peridotites, suggesting that our harzburgites cannot be explained as residues of anhydrous adiabatic melting and melt-rock interaction at mid-ocean ridges alone. This implies that the petrogenesis of SE Turkey harzburgites involves additional processes and components. Harzburgites with the most depleted heavy-rare earth element (HREE) contents are the ones with the highest abundance of strongly incompatible elements, which can be explained by open-system processes where the peridotites in the mantle wedge experienced melting and infiltration of enriched components simultaneously. Open-system dynamic melting models with continuous flux of sediment-derived melts can account for the observed correlation, but are numerically too low compared to the measured values. Based on the observed fractionation between Zr, Hf, and elements with similar incompatibility (middle REEs), we hypothesized the involvement of amphibolite-derived melt and modeled its numerical trace-element contents. Binary mixing between this hypothetical melt and residues of the former open-system model can coherently account for the majority of the obtained trace-element data. This indicates that magmatic events during subduction initiation likely involve multiple components and occur in multiple stages, and that melt-mantle interaction plays a significant role in oceanic forearc lithosphere formation. Based on our model, we suggest the high (Zr/MREE)N signatures in some boninites and depleted harzburgites found in modern forearcs and ophiolites could be inherited from amphibolite-derived melts. Moreover, the existence of slab melts agrees with current constraints on the reconstructed geothermal gradients during subduction initiation based on the petrology and geochemistry of metamorphic soles.

Petrogenesis of the Nashwaak Granite, West-Central New Brunswick, Canada: Evidence from Trace Elements, O and Hf Isotopes of Zircon, and O Isotopes of Quartz

The petrogenesis of the Pridoli to Early Lochkovian granites in the Miramichi Highlands of New Brunswick, Canada, is controversial. This study focuses on the Pridoli Nashwaak Granite (biotite granite and two-mica granite). In situ trace elements and O and Hf isotopes in zircon, coupled with O isotopes in quartz, are used to reveal its magmatic sources and evolution processes. In the biotite granite, inherited zircon cores have broadly homogenous δ18OZrc ranging from +6.7‰ to 7.4‰, whereas magmatic zircon rims have δ18OZrc of +6.3‰ to 7.2‰ and εHf(t) of −0.39 to −5.10. The Hf and Yb/Gd increase with decreasing Th/U. Quartz is isotopically equilibrated with magmatic zircon rims. The biotite granite is interpreted to be solely derived by partial melting of old basement rocks of Ganderia and fractionally crystallized at the fO2 of 10−21 to 10−10 bars. The two-mica granite has heterogeneous inherited zircon cores (δ18OZrc of +5.2‰ to 9.9‰) and rims (δ18OZrc of +6.2‰ to 8.7‰), and εHf(t) of −11.7 to −1.01. The two-mica granite was derived from the same basement, but with supracrustal contamination. This open-system process is also recorded by Yb/Gd and Th/U ratios in zircon and isotopic disequilibrium between magmatic zircon rims and quartz (+10.3 ± 0.2‰).

Temporal Variations of Petrological Characteristics of Tangkil and Rajabasa Volcanic Rocks, Indonesia

DOI:10.17014/ijog.7.2.135-159Tangkil and Rajabasa Volcanoes are neighbouring subduction-zone volcanoes located on the southeast tip of Sumatra Island (Sunda Strait, Indonesia). Stratigraphic correlation of lavas in Tangkil-Rajabasa volcanic area was established from field observations, morphological analysis, and K-Ar dating analysis. Detailed petrography and geochemical data of two and eleven lava units from Tangkil and Rajabasa, respectively, were then integrated with the stratigraphy to show the temporal variations. Early stage (> 4.3 Ma) effusives of Tangkil Volcano are dacitic to rhyolitic (67-71 wt. % SiO2; Tklf), whereas the later (c. 4.3 Ma) rocks are basalt to basaltic andesite (c. 52 wt. % SiO2; Tklm). Tangkil shows bimodal magmatism, of which the felsic endmember is > 71 wt. % SiO2 and < 0.1 wt. % MgO. Lavas of Rajabasa Volcano are comparatively younger (c. 0.3 to 0.1 Ma) with compositions ranging from basalt to andesite (51-62 wt. % SiO2; Rbs). Chemical variations of Rajabasa accounts for the interactions of at least three endmembers: Mg-rich medium-K basalt magma, low-Mg medium-K basalt magma, and high-K andesitic magma. During the long evolution of Rbs magma system, the temporal chemistry shows rising-falling variation in SiO2 and MgO indicating the three magmas were active. The felsic endmember magma of Rajabasa is fixed in composition (at ~62 wt. % SiO2; ~2.2 wt. % MgO). The rocks from the last Tklf and Rbs indicate open system processes by containing plagioclase and pyroxene phenocrysts that show resorption of evolved core and overgrowth of less evolved mantle. The multiple zones of dissolution-overgrowth in plagioclase crystals and the fluctuating trend in temporal whole-rock variation suggest that the changes of magmatic condition in temperature, H2O, or chemical composition were repetitive.

Chemical zoning and open system processes in the Laacher See magmatic system

Unravelling the generation of compositionally and thermally zoned magma reservoirs is important to our understanding of the dynamic processes operating in magmatic systems. Here, we present new major and trace element data for volcanic glasses from the classically zoned Laacher See Tephra and suggest that mafic recharge may play an important role in producing the observed compositional gradient. Mafic phonolite glass from the upper part of the Laacher See Tephra records the addition of ca. 30% basanite magma, which is recognised by an increase in REE + Y and a decrease in Th, U and Zr relative to glasses from more evolved units. We suggest that the Laacher See magmatic system was sustained by repeated episodes of basanite recharge and calculate a recharge magma flux of between 2 × 10–5 and 1 × 10−4 km3y−1 in the 20 kyr leading up to the eruption. Basanite addition would have provided heat required to generate the strong compositional and thermal gradients that are recorded in crystals ejected during the Laacher See Tephra eruption.

Geochemical and isotopic constraints on the evolution of magma plumbing system at Damavand Volcano, N Iran

Damavand is a Quaternary stratovolcano in N Iran, associated with small volumes of under-saturated to weakly-saturated mafic lavas (Tephrite-Basanite-Alkali olivine basalt), and large volumes of saturated lavas (trachyandesite-trachyte (TT)) together with minor pyroclastic rocks. TT rocks are characterized by LILE and HFSE enrichments, whereas the mafic rocks have higher REE and other incompatible elements (except Rb, Th) than TT suite rocks, contradicting the assumed derivation of TT magmas from mafic magmas by simple fractional crystallization processes. The similar whole-rock isotope ratios of mafic rocks, TT suites and their enclaves do suggest a common origin. Magmatic enclaves in TT suites show decoupled major and some trace elements from their host while they have similar geochemistry to the mafic lavas. Trace element modeling including assimilation-fractional crystallization (AFC) and energy-constrained recharge-assimilation fractional crystallization (EC-RAFC) suggest that the Damavand TT rocks were formed by multi-stage fractionation of mafic magma with minor effects of open system processes. Differentiation started at high pressures (6–8 kbar, initial temperature of magma (Tm) = 1180 °C) involving anhydrous minerals (mainly clinopyroxene-orthopyroxene), accompanied by recharge and wall-rock assimilation at equilibration temperature (Teq) of 1050 °C as the magma reached the threshold for extraction from crystal mush (Mm = 0.49) (stage-1). These processes resulted in the enrichment of the residual liquid in incompatible elements. The fractionation of oligoclase-andesine, anorthoclase, phlogopite and apatite from intermediate enriched melts occurred at shallow crustal levels (0.5–3 kbar, Tm = 1050 and Teq = 830 °C): here, FC processes and further recharge and assimilation of crustal materials reduced LILE and REE concentration in the residual liquid (stage-2). The role of enriched melts is supported by unusual trace element composition of key minerals, for instance, feldspars have variable concentrations of Ba (132–5495 ppm), Sr (1471-14413 ppm) and unusual Rb, La and Ce contents. Based upon melt extraction modeling, physical removal of residual melts at stage-2 could produce cumulate residues with trace element compositions comparable to enclaves in TT suites (stage-3). Disruption of crystal mush due to recharge, followed by mixing of enclave material with evolved trachytic melts caused buffering of magma at trachyandesitic composition (stage-4). The results affirm the advantages of using a range of compositional and textural records in modeling of petrogenetic processes in all magmatic plumbing systems.

Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust

The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction.

Reactive melt migration controls the trace element budget of the lower oceanic crust: insights from the troctolite-olivine gabbro association of the Pineto Ophiolite (Corsica, France)

The Pineto gabbroic sequence is a ~ 1.5 km-thick crustal section similar to those forming the oceanic core complexes at modern slow-spreading ridges. The sequence is subdivided into two sectors displaying different bulk compositions. The deeper sector consists of troctolites, with minor olivine gabbros and sparse olivine-rich troctolite intercalations. We carried out a thorough chemical characterization, including minerals and bulk rocks, of the troctolite/olivine gabbro association. Although the whole-rock compositions are mainly controlled by mineral proportions, the whole-rock variability of some minor (e.g., Cr, Ni) and trace (e.g., Y, REE) elements reveals formation by open-system crystallization processes. This interpretation is supported by the trace element mineral compositions. The incompatible trace element signature of clinopyroxene in the different rock types is nearly undistinguishable, whereas olivine and plagioclase show substantial variability of trace element compositions. In particular, olivine and plagioclase from the Ol-gabbros are typically depleted in incompatible elements compared to olivine and plagioclase from the troctolites. This chemical variability attests that fractional crystallization alone cannot have determined the compositions of these rocks, but requires an open system process where melts migrate and chemically interact with pre-existing crystal mushes leading to the redistribution of major and trace elements throughout the cumulate pile.

The Mount Cameroon southwest flank eruptions: Geochemical constraints on the subsurface magma plumbing system

With seven major eruptions in the 20th century, Mount Cameroon is one of the most active volcanoes in Africa. However, information on the volcano's plumbing system is still relatively scarce. In order to contribute to filling this knowledge gap, a geochemical, clinopyroxene geobarometry, and SrNd isotope study was carried out on samples collected from the 1922 and lower 1999 lava flows found on the SW flank of the volcano. Petrographic evidence shows a common crystallising mineral assemblage of olivine and clinopyroxene and/or plagioclase scattered in a microlite-rich groundmass. The total alkali-silica classification diagram shows that the lavas of 1922 exhibit basanitic compositions those of 1999 are predominantly basanites with some few hawaiites. The Sr isotope ratios for the 1999 samples in this study show values somewhat lower and higher than the limits obtained for historic Mount Cameroon samples so far. While major and trace element trends indicate fractional crystallisation processes, chemical and mineral disequilibrium textures are indicative of open-system processes such as magma mixing and magma chamber recharge. Variations in incompatible trace element ratios, which hardly change during fractional crystallisation, are also suggestive of open-system processes. Plots of K2O and Nb show a mixing trend between samples from these eruptions, implying a common magma chamber where remnant material from preceding eruptions was incorporated into succeeding eruptions. Clinopyroxene geobarometric calculations show this common dominant crystallisation depth at 23–29 km.

Isotopic Compositions of Sulfides in Exhumed High‐Pressure Terranes: Implications for Sulfur Cycling in Subduction Zones

AbstractSubduction is a key component of Earth's long‐term sulfur cycle; however, the mechanisms that drive sulfur from subducting slabs remain elusive. Isotopes are a sensitive indicator of the speciation of sulfur in fluids, sulfide dissolution‐precipitation reactions, and inferring fluid sources. To investigate these processes, we report δ34S values determined by secondary ion mass spectroscopy in sulfides from a global suite of exhumed high‐pressure rocks. Sulfides are classified into two petrogenetic groups: (1) metamorphic, which represent closed‐system (re)crystallization from protolith‐inherited sulfur, and (2) metasomatic, which formed during open system processes, such as an influx of oxidized sulfur. The δ34S values for metamorphic sulfides tend to reflect their precursor compositions: −4.3 ‰ to +13.5 ‰ for metabasic rocks, and −32.4 ‰ to −11.0 ‰ for metasediments. Metasomatic sulfides exhibit a range of δ34S from −21.7 ‰ to +13.9 ‰. We suggest that sluggish sulfur self‐diffusion prevents isotopic fractionation during sulfide breakdown and that slab fluids inherit the isotopic composition of their source. We estimate a composition of −11 ‰ to +8 ‰ for slab fluids, a significantly smaller range than observed for metasomatic sulfides. Large fractionations during metasomatic sulfide precipitation from sulfate‐bearing fluids, and an evolving fluid composition during reactive transport may account for the entire ~36 ‰ range of metasomatic sulfide compositions. Thus, we suggest that sulfates are likely the dominant sulfur species in slab‐derived fluids.

Magma chamber processes and dynamics beneath northwestern Anatolia: Insights from mineral chemistry and crystal size distributions (CSDs) of the Kepsut volcanic complex (NW Turkey)

Mineral chemistry, petrography and crystal size distributions of Early Miocene mafic and intermediate lavas from the Kepsut Volcanic Complex (KVC) have been studied to decipher magma chamber processes and dynamics beneath northwestern Anatolia. KVC which is a part of the Western Anatolian Volcanic Province comprises lavas represented by various compositions ranging from basaltic andesite to rhyodacite and associated pyroclastics. Basaltic andesite and andesite lavas of the KVC, the focus of this study, present distinct petrographical features such as three plagioclase generations, patchy zoning, oscillatory reverse zoning, sieve textured crystals and clinopyroxene-mantled amphibole that are clear indicators of disequilibrium conditions in magma chamber(s). Petrography, mineral chemistry and crystal size distribution studies of andesite and basaltic andesite lavas of the KVC indicate that magma evolution occurred in four stages: (I) Near-Equilibrium Stage, (II) Mafic Input Stage, (III) Mixing/Mingling Stage and (IV) Pre-eruption Stage. Thermobarometry calculations indicate that crystallization of andesite and basaltic lavas occurred at similar pressure (mid-to-deep crustal depths, 5–7 kbar) but distinct temperature conditions (920–952 °C and 921–1069 °C, respectively) suggesting that KVC magmas evolved either in the same or different magma chambers at similar depths and experienced open-system processes. Both of these hypotheses are compatible with multistage evolution of the KVC. Combined petrography, mineral chemistry and geothermometry studies on KVC rocks indicate that mixing/mingling processes and subsequent decompression-driven crystallization were the principal mechanisms for the origin of the textural and mineralogical diversity that is characteristic of KVC lavas. Disequilibrium crystallization was probably caused by the influx of a hotter basaltic magma into crustal magma chambers during the first major period of north-south extension in western Anatolia.

Crustal contamination and genesis of transitional alkaline-tholeiitic intrusions: Insights from the José Fernandes Suite, Paraná Magmatic Province, Brazil

The José Fernandes suite, located in the Ponta Grossa Arch (PGA) region of southern Brazil, is an approximately 3 km2 composite intrusion associated with the Early Cretaceous (134 Ma) intrusive magmatism that formed the Paraná-Etendeka Magmatic Province. The intrusion is emplaced in Precambrian metasedimentary rocks and composed of gabbroic rocks (including a marginal quartz-bearing variety) intruded by synplutonic alkaline dikes. Petrographic and geochemical characteristics indicate the important influence of crustal contamination on the evolution of the suite. Disequilibrium textures and mineral chemical zoning are widespread in the rocks composing the pluton and are evidence of open-system processes. Orthopyroxene (Fe-enstatite) (Mg# 49-68) occurs in reaction rims of iron-rich olivine (Mg# 42-61) and symplectic intergrowths, together with magnetite and ilmenite. The presence of Fe-enstatite can be related to partial melting of host rocks resulting in non isochemical reactions of olivine with fluids under oxidizing conditions. The positive correlation of SiO2 and Sr—Pb isotope initial ratios and negative correlation with εNd indicate different levels of crustal contamination. The alkaline dikes that cut the pluton have basanite to basaltic trachyandesite compositions and vary from primitive to contaminated (87Sr/86Sri = 0.7049 to 0.7070) dikes, confirming that the intrusion formed from several batches of alkaline magma with distinct crustal contributions. Given the enrichment in K2O over Na2O in samples with high initial 87Sr/86Sr (>0.707) and evidence of a low time-integrated Th/U component typical of the upper crust, metapelitic country rocks are considered potential contaminants in the upper crustal chamber environment. A pole of contamination with low 143Nd/144Nd signatures indicates contamination by gneissic basement with Archean crustal residence at lower crustal levels. Isotopic modeling is consistent with an evolution in which several batches of basanite magma evolved to more radiogenic compositions by assimilation of different proportions (up to ~35%) of partial melts from different crustal contaminants. These contaminants have older crustal residence times than those involved in the generation of silicic magmas associated with the tholeiitic basalts of the Paraná-Etendeka Province.

Building Arc Crust: Plutonic to Volcanic Connections in an Extensional Oceanic Arc, the Southern Alisitos Arc, Baja California

AbstractThe ∼50 km long Rosario segment of the Cretaceous Alisitos oceanic arc terrane provides undeformed three-dimensional exposures of the upper 7 km of an oceanic extensional arc, where crustal generation processes are recorded in both the volcanic and underlying plutonic rocks. These exceptional exposures allow for the study of the physical and chemical links between the rock units and help constrain the differentiation processes active during the growth and evolution of arc crust. This study focuses on the southern third of the Rosario segment, previously referred to as the southern volcano-bounded basin, and its plutonic underpinnings. Upper crustal rocks in the Rosario segment consist of a 3–5 km thick volcanic–volcaniclastic section with hypabyssal intrusions. Plutons intrude these units at various levels along-strike, but at each intrusive contact the transition is complete over a distance of <150 m, where stoped volcanic blocks are present. There is striking compositional overlap in whole-rock and mineral chemistry between the plutonic and volcanic units, suggesting a comagmatic source. Whole-rock geochemistry shows coherent trends in major and trace elements in mafic to intermediate compositions, but less coherent trends above 63 wt % SiO2. Units are predominantly low-K with flat rare earth element patterns, and show large ion lithophile element enrichment and high field strength element depletion. Initial Nd and Pb isotope ratios overlap for all units and imply no cratonic continental involvement. This agrees with low Sr/Y ratios of all rock types, indicative of thin, immature oceanic arc crust. Modeling results show that closed-system fractional crystallization drove crustal differentiation from mafic to intermediate compositions, but open-system processes likely occurred to produce some of the felsic compositions. Differentiation occurred in a two-step fractionation process. Step 1, from basaltic andesite to andesite, fractionated an anhydrous gabbroic cumulate (∼40% crystallization). Step 2, from andesite to rhyolite, fractionated a hydrous amphibole cumulate (∼65% crystallization, total), which is similar to what fluid dynamical models suggest for production of rhyolite (between 50–70% crystallization). Our results can be used as a reference model for differentiation processes relating to the growth of the middle and upper crust within active extensional arc systems. The Rosario segment plutonic rocks may be analogous to the low-velocity zone (Vp = 6·0–6·5 km s–1) imaged within the extensional Izu–Bonin arc. The chemistry of the plutonic and volcanic rocks is most similar to those of volcanic rocks in the Izu–Bonin active rift.

Quantitative Data Extraction from Orthopyroxene Trace Element Maps and Its Potential to Examine the Formation of the UG2 Unit, Bushveld Complex

We present a mineral chemical stratigraphy across the UG2 Unit, situated in the Upper Critical Zone of the Bushveld Complex. The magmatic evolution of the rocks hosting this world-class ore deposit for chromium and platinum group elements (PGEs) is still under debate. Orthopyroxene is a common phase across the magmatic stratigraphy of the unit. It hosts trace metals, whose relative contents can record information about magmatic processes. This study determined laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) trace element data for orthopyroxene and spot LA multi-collector ICP-MS Sr-isotope data for plagioclase. Mapping of trace element distributions revealed internal zoning in Cr, V, Y, and Th. Unlike in conventional spot analysis, quantitative trace element concentrations were extracted post-analysis from two-dimensional LA-ICP-MS maps. This approach offered advantages over spot analysis. The maps revealed internal complexities (e.g., inclusions, exsolved phases, zonations and diffusion halos) that could be avoided with suitable thresholds for chemical filter elements (i.e., 100 ppm < Cr < 4000 ppm, Y < 6 ppm), permitting fast, automated extraction of pure orthopyroxene data pixels from elemental maps covering noritic, troctolitic, anorthositic and chromititic mineral assemblages. Diffusive equilibration in orthopyroxene was evaluated by comparing zonations in elements with different known diffusivities. Magnesium and Ni are homogenously distributed, indicative of rapid diffusive processes, while Cr and V show zonations, interpreted to largely reflect primary distribution. Thorium and Y apparently diffused along discrete pathways, indicated by concentration anomalies following orthopyroxene lamellae. A systematic study of the trace element inventory of orthopyroxene across the succession of the UG2 Unit, complemented with Sr isotope signatures of plagioclase, revealed clear evidence for open-system processes: compatible elements (Cr, Ni) are decoupled from Mg# variations in orthopyroxene from specific UG2 subunits; distinct Mg# peaks coincide with negative V excursions in orthopyroxene from chromite-bearing subunits; towards the top of the unit, plagioclase shows a distinct 87Sr/86Srinitial peak and orthopyroxene shows Cr-depleted and Th-enriched rims. These chemical features cannot be explained by crystal mixing alone and argue for the involvement of at least two chemically distinct melts in the formation of the studied UG2 section.

Improved constraints on open-system processes in fossil reef corals by combined Th/U, Pa/U and Ra/Th dating: A case study from Aqaba, Jordan

Here we present 230Th/U, 231Pa/U as well as 226Ra/230Th isotope ratios from five fossil reef corals of Last Interglacial origin from the Gulf of Aqaba, Northern Red Sea. The results show clear evidence for open-system behaviour with strongly elevated δ234U values and U concentrations indicating post-depositional U addition. The combined application of all isotope systems enables us to better constrain the nature and timing of the open-system processes than only based on the 230Th/U data.Quantitative modelling of the diagenetic processes allowed us to reproduce the trends in the isotope ratios. Two of the five corals were probably affected by two separate phases of U addition with different δ234U values. The trends observed for two other corals can be explained by U addition followed by U loss. The fifth coral shows signs of both U gain and loss at the same time in the more recent past. The timing of the diagenetic processes is remarkably similar for the five corals and can be constrained to approx. 1 and 6 thousand years (ka) and 100 and 102 ka after coral growth, respectively.Based on the modelling results, we suggest that conventional 231Pa/230Th ages provide the best estimate for the true age of four of the five corals, which range from 109.1 to 114.1 ka. This implies a late Last Interglacial time of deposition. For the fifth coral, the most reliable age estimate is the conventional 230Th/U age of one of the subsamples (117.3 ka), based on a concordia diagram for all subsamples.The timing of the modelled open-system processes suggests that the early event of U addition was associated with interaction of the corals with 234U-enriched seawater or saline groundwater. The later open-system event can be described as U redistribution within the coral reef, since some corals apparently lost U while others gained U. The timing of the second event is broadly consistent with the Bølling-Allerød interstadial, which was probably characterised by enhanced wetness in this typically hyper-arid region.

Open-system magmatic behaviour beneath monogenetic volcanoes revealed by the geochemistry, texture and thermobarometry of clinopyroxene, Kaikohe-Bay of Islands volcanic field (New Zealand)

The ascent and temporary crustal storage of magmas beneath basaltic monogenetic volcanoes in continental settings is poorly understood, but is important in the study of their petrogenesis, and for monitoring future volcanic unrest. The Kaikohe-Bay of Islands volcanic field in northern New Zealand comprises Quaternary-aged monogenetic basalt volcanoes in an intraplate setting. Clinopyroxene phenocrysts in the basalts provide an opportunity to investigate the ascent history of the magmas that may not be evident from studies at the scale of whole-rock samples. The phenocrysts are texturally and compositionally diverse, and demonstrate the dominance of open system processes such as magma mixing and crystal entrainment in the crust. Many phenocrysts have an Mg (Mg# ~80–83) and Cr rich rim overgrowth which is in equilibrium with the host rock, but a resorbed, core (Mg# ~70) that crystallised from a more evolved magma. These crystals record mafic recharge, presumably the trigger to eruption. Other crystals are characterised by a high-Mg (Mg# ~82–84) and Cr core surrounded by a disequilibrium, low-Mg (Mg# 70–75) rim. The rims are reversely zoned and demonstrate late-stage equilibrium with the host rock. These crystals either nucleated in the host magma or were entrained from earlier basaltic intrusions of similar composition. The rim zones record re-melting of intrusions, or interaction with more differentiated magmas, followed by mafic recharge. Subordinate crystal types include oscillatory zoned crystals (Mg# >80) that nucleated in the ascending magma, and diffusely, patchy-zoned crystals that are variously antecrystic or xenocrystic in origin. Cognate gabbro inclusions contain clinopyroxene in equilibrium with the host basalt, suggesting they originated from side-wall crystallisation. Crystal-melt equilibria indicate that the cognate clinopyroxene formed at 430 ± 170 MPa (2σ) (=16 ± 6 km depth), and a subordinate population formed at 730 ± 160 MPa (=28 ± 6 km depth). These depths coincide with major seismic velocity contrasts at a zone of partial melt (10–19 km) and the Moho (~28 km). Thus, buoyancy or rheology contrasts in the crust temporarily slowed magma ascent and promoted periods of crystallisation and the assimilation of antecrystic and xenocrystic components. Similar magmatic processes occurred at the neighbouring and contemporaneous Whangarei volcanic field. Both of these fields have been active for millions of years, and longevity could explain the formation of differentiated crustal instrusives. In systems where magmas temporarily stall, there is a greater likelihood of detecting pre-eruption geophysical phenomena that could act as signals to pending eruptions.

Boson–boson pure-dephasing model with non-Markovian properties

In this paper, we discuss the mechanism of pure-dephasing process with a newly proposed boson–boson model, namely, a bosonic field coupled to another bosonic bath in thermal equilibrium. Our model is fully solvable and can reproduce the pure-dephasing process which is usually described by the well-known spin–boson model, therefore offering a new perspective to understanding decoherence processes in open quantum systems of high dimension. We also show that this model admits a generically non-Markovian dynamics with respect to various different non-Markovian characterizations, i.e., the criteria based on divisibility, quantum regression formula and Wigner function, respectively. The criterion based on Wigner function is firstly proposed in this paper. For the case that the particle number of the pure-dephasing system is constrained to be 0 or 1, we analytically prove its equivalence to the criteria based on trace distance and divisibility.

Decoding magma storage and pre-eruptive processes in the plumbing system beneath early Carboniferous arc volcanoes of southwestern Tianshan, Northwest China

The volcanic rocks, consisting of basalt, basaltic andesite, andesite, and tuff with volcanic clastic sedimentary rocks in the Dahalajunshan region represent an early Carboniferous continental arc formed during subduction of the Paleo-southern Tianshan ocean. The basalt is characterized by a phenocryst assemblage of clinopyroxene and plagioclase, while basaltic andesite and andesite contain variable amounts of clinopyroxene, orthopyroxene, plagioclase, and FeTi oxides phenocrysts. Clinopyroxene, the most common phenocryst in the studied rocks, has a large variation of compositions and displays multiple zoning patterns, suggesting complex magmatic processes in different magma storages beneath the Dahalajunshan region. A deep reservoir at depths of >28 km is suggested by clinopyroxene with Mg# values of 84–88 and an intermediate reservoir is suggested by clinopyroxene with Mg# values of 75–79. The shallow reservoir is crystal-rich as indicated by the presence of crystal clots consisting of Fe-rich clinopyroxene (Mg# < 75), orthopyroxene (Mg# = 65–76), plagioclase, and FeTi oxides. Petrological observations, combined with the whole-rock geochemical and SrNd isotopic data, suggest that the studied basalt mainly formed by fractional crystallization in both deep and intermediate magma reservoirs, while basaltic andesite and andesite might be produced via open-system processes of crystal fractionation, recycling of early-formed crystals, interaction of colder magmas with hotter, recharging magmas, and crustal contamination in the shallow reservoir. This study suggests the importance of textures and compositions of erupted minerals to decode the pre-eruptive histories of magmatic plumbing systems beneath fossil arc volcanoes.

Geochemical and textural constraints on degassing processes in sub-Plinian eruptions: case-study of the Greenish Pumice eruption of Mount Somma-Vesuvius

Plinian eruptions are characterized by high intensity and an overall steady character, and result in a stable convective column. The main processes controlling the dynamics of such steady and stable plume systems have been extensively investigated. Conversely, sub-Plinian eruptions are unsteady, as recorded by the large variability of the products and deposits. Our knowledge of the processes creating this unsteadiness on various timescales remains limited, and still requires more observations as well as theoretical and experimental investigation. Here, we focus on the sub-Plinian eruption of the Greenish Pumice (GP, 19,265 ± 105 BP), Mt. Somma-Vesuvius (Italy). On the basis of coupled geochemical and textural analyses of samples from the well-established stratigraphy of the GP deposits, we investigate volatiles (H2O, CO2, F, Cl) to better constrain the unsteady sub-Plinian eruptive style. This allows us to carry out a detailed study of the degassing processes in relation to the eruption dynamics. We find that degassing by open-system processes generally dominates throughout the entire eruption, but alternates with episodes of closed-system degassing. The fluctuating degassing regimes, responsible for the variable magma ascent rate within the conduit, are also responsible for the eruptive column instability. Volatile behavior is well correlated with textural heterogeneities of the eruptive products. Both reflect higher conduit heterogeneity than for Plinian eruptions, where we find a higher horizontal gradient in magma ascent velocity due to a smaller conduit diameter.

Sr isotope zoning in plagioclase from andesites at Cabo De Gata, Spain: Evidence for shallow and deep contamination

Plagioclase crystals in andesites from the Cabo De Gata region show generally radiogenic Sr isotope compositions and consistent core to rim increases in 87Sr/86Sr that are indicative of open system processes in the lithosphere and crustal contamination during crystallization. High-grade metamorphic rocks of the Alpujárride and Nevado-Filábride complexes represent the most likely crustal contaminants. The plagioclases are characterized by subtly zoned and resorbed calcic cores (An73–86). These cores also have radiogenic 87Sr/86Sr (0.7127–0.7129), although typically less radiogenic than plagioclase rims, groundmass plagioclase and whole rock compositions (up to 87Sr/86Sr = 0.7135). These cores are interpreted to represent early crystallization of plagioclase from hydrous melts emplaced into the lower crust. The parental melts to these andesites must therefore have already inherited their radiogenic Sr isotope compositions prior to entering the lower crust and before the onset of crystallization of plagioclase, which is inconsistent with previous models suggesting that the generally radiogenic nature of Sr in these volcanics reflects large amounts of crustal contamination. Instead, the isotope systematics are consistent with models invoked significant addition of a subducted sediment component to the mantle source. The high-An% plagioclase cores are characterized by resorption textures, which are consistent with dissolution during rapid decompression and/or devolatisation during magma migration from the lower crust into upper crustal magma chambers.