Variations in Earth's 1D viscosity structure in different tectonic regimes

Earth’s long-wavelength geoid provides insights into the thermal, structural, and compositional evolution of the mantle. Historically, most estimates of mantle viscosity using the long-wavelength geoid have considered radial variations with depth in a symmetric Earth. Global estimates of this kind suggest an increase in viscosity from the upper mantle to lower mantle of roughly 2 -- 3 orders of magnitude. Using a spatio-spectral localization technique with the geoid, here we estimate a series of locally constrained viscosity-depth profiles covering two unique regions, the Pacific and Atlantic hemispheres, which show distinct rheological properties. The Pacific region exhibits the conventional Earth's 1D rheology with a factor of roughly 80-100 increase in viscosity occurring at transition zone depths (400 - 800 km). The Atlantic region in contrast does not show significant viscosity jumps with depth, and instead has a near uniform viscosity in the top 1000~km. The inferred viscosity variations between our two regions could be due to the prevalence of present-day subduction in the Pacific and the infrequence of slabs in the Atlantic, combined with a possible hydrated transition zone and mid-mantle of the Atlantic region by ancient subduction during recent supercontinent cycles. Rigid slab material within the top 800 km, with about 90\% Majoritic garnet in the form of subducted oceanic crust, coupled with unique regional mantle structures, may be generating a strong transition zone viscosity interface for the Pacific region. These effective lateral variations in mantle viscosity could play a role in the observed deformation differences between the Pacific and Atlantic hemispheres.

Rehydration Driven Na-Activation of Bentonite—Evolution of the Clay Structure and Composition

A new method of Na-activation of raw bentonite, rich in Ca-montmorillonite, consisting of combined thermal treatment at 200 °C, followed by immediate impregnation with aqueous solution of Na2CO3 of concentration corresponding to 0.5, 1.0, 1.5, or 2.0 cation exchange capacity (CEC) of clay, was investigated. Structural and compositional evolution of the activated solids after 1, 2, 3, and 4 weeks of storage was monitored by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). XRD analysis indicated that within the investigated period of ageing transformation to Na-rich montmorillonite required Na2CO3 concentration of at least 1.0 CEC. FTIR spectra showed that, depending on the Na2CO3 concentration and ageing time, formation of Na-rich montmorillonite was accompanied by precipitation of poorly crystalline calcite, amorphous calcium carbonate, gaylussite (a double calcium-sodium carbonate), and portlandite (Ca(OH)2).

Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems

Crystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation of mantle-derived arc magmas. Various pressure–temperature trajectories were explored to cover a range of potential magma ascent paths and to investigate the role of decompression on phase equilibria and liquid lines of descent (LLD). Fractional crystallisation was approached in a step-wise manner by repetitively synthesising new starting materials chemically corresponding to liquids formed in previous runs. Experiments were performed at temperatures ranging from 1140 to 870 °C with 30 °C steps, and pressure was varied between 0.8 and 0.2 GPa with 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO2) was buffered close to the Ni-NiO equilibrium (NNO). An additional fractionation series was conducted at fO2 corresponding to the Re-ReO2 buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) were run in piston cylinder apparatus while 0.2 GPa runs were conducted in externally heated pressure vessels. Resulting liquid lines of descent follow calc-alkaline differentiation trends where the onset of pronounced silica enrichment coincides with the saturation of amphibole and/or Fe–Ti–oxide. Both pressure and fO2 exert crucial control on the stability fields of olivine, pyroxene, amphibole, plagioclase, and Fe–Ti–oxide phases and on the differentiation behaviour of arc magmas. Key observations are a shift of the olivine–clinopyroxene cotectic towards more clinopyroxene-rich liquid composition, an expansion of the plagioclase stability field and a decrease of amphibole stability with decreasing pressure. Decompression-dominated ascent trajectories result in liquid lines of descent approaching the metaluminous compositional range observed for typical arc volcanic rocks, while differentiation trends obtained for cooling-dominated trajectories evolve to peraluminous compositions, similar to isobaric liquid lines of descent at elevated pressures. Experiments buffered at RRO provide a closer match with natural calc-alkaline differentiation trends compared to fO2 conditions close to NNO. We conclude that decompression-dominated fractionation at oxidising conditions represents one possible scenario for arc magma differentiation.

The Oruanui Eruption: Insights into the Generation and Dynamics of the World's Youngest Supereruption

<p>This work investigates the pre- and syn-eruptive magmatic processes that culminated in the world’s youngest supereruption – the ~25.4 ka, 530 km³ Oruanui eruption from Taupo volcano, New Zealand – from the perspective of crystals contained in single parcels of frozen magma (pumice). The eruption is unusual in its variety of magmatic compositions. About 98-99 % by mass of the juvenile material is high-SiO₂ rhyolite (HSR; >74 wt% SiO₂), with lesser volumes of tholeiitic and calc-alkaline mafic magmas (total 3-5 km³; basaltic andesite to andesite: 53-63 % SiO₂), low-silica rhyolite (LSR: 0.1-0.5 km³; <74 wt% SiO₂) and a ‘foreign’ biotite-bearing rhyolite from an adjacent magma source (0.03 km³; ~74 wt% SiO₂). Detailed textural and chemical data from amphibole, plagioclase, and orthopyroxene are placed within the context of an established time-stratigraphic, volcanological and petrographic framework, of unrivalled detail globally for an eruption of this age and magnitude. Other previously published information from zircon and quartz is also incorporated. This unique contextual information is used to constrain observations and inferences regarding the processes that moved the Oruanui magma from a largely uneruptible crystal-rich progenitor at depth (where an eruption was possible), to a highly eruptible melt-rich magma at shallow crustal levels (where eruption was inevitable). A thermally and compositionally stratified crystal mush body, with an upper SiO₂-saturated and quartz bearing cap at ~3.5 km depth and quartz-free roots extended down to at least ~10 km. This inference is made on three bases. 1) That the quartz cores contain trapped melt that is more evolved than the melt component of the immediately pre-eruptive magma body, indicating their growth within mush from a more evolved interstitial melt. 2) The majority of plagioclase, amphibole, and orthopyroxene cores, in contrast to quartz have compositions that indicate growth from less evolved melts than that encountered in the final melt-dominant magma body. 3) Barometric estimates from amphibole core compositions indicate derivation from a range of depths (~3.5 to 10 km). The spatial and temporal transitions from mush to melt-dominant magma body are recorded in the textural and compositional zonations within the crystal phases. Crystals from all levels of the zoned mush body were entrained during the melt extraction process resulting in a diversity of crystal compositions being brought together in the melt-dominant magma body. Textural disequilibrium features in the cores of orthopyroxene and plagioclase crystals reflect their temporary departure from stability during the accompanying significant decompression (recorded in the amphibole model pressures). Counterpart chemical signatures, reflecting this partial orthopyroxene and plagioclase dissolution, are recorded in the amphiboles which show no textural evidence for destabilisation during ascent. Crystal chemical and textural zonation in the rim growths of the plagioclase, orthopyroxene, and amphibole record further crystallisation in the accumulating melt-dominant magma body, and reflect cooling and compositional evolution of the body towards its final pre-eruptive conditions. The timing of growth of the melt dominant magma body is constrained by Fe-Mg diffusion modelling of key boundaries in orthopyroxene crystals. Accumulation of this body began only ~1600 years and peaked at 230 years prior to the eruption, as vast volumes of melt and entrained crystals were drained from the mush body and began to accumulate at shallower levels (~3.5 to 6.0 km depth). Within the thin, sill-like melt-dominant magma body, significant heat loss drove vigorous convection. Textural and chemical zonation patterns within the rim-zones of plagioclase, orthopyroxene and amphibole, inferred to have grown solely in the melt-dominant magma body, depict a secular cooling and melt evolution trends towards final uniform thermal (~770 °C) and compositional conditions inferred for the HSR magma. Despite the rapid accumulation of a vast volume of crystal-poor HSR magma at shallow crustal levels, the apparent gas-saturated nature of that magma, and vigorous convection within the melt-dominant magma body itself, the chronologies from HSR orthopyroxene imply that the magma underwent a period of stasis of about 60 years. The presence of 3-16 wt% of ‘foreign’ biotite-bearing juvenile pumices in the early Oruanui fall deposits (phases 1 and 2) show that coincident with the onset of the Oruanui eruption, magma was transported laterally in a dike from an adjacent independent magma system 10-15 km to the NNE to intersect the active Oruanui conduit. Consideration of the tectonic stress orientations associated with this lateral transport imply that an external tectonic influence through a major rifting event was a critical factor in the initiation of the Oruanui eruption. Only the presence of the foreign magma, and linkages to detailed field-based and geochemical constraints enables the tectonic influence to be identified. During the eruption itself, minor quantities of Oruanui LSR magma were erupted , and with a crystal cargo, reflecting derivation from deeper (mostly >6 km), hotter (~820 °C) sources in the crystal mush roots to the system. Comparisons of LSR crystal compositions with cores to many HSR crystals for plagioclase, orthopyroxene and amphibole imply that the LSR magma was derived from pockets in the mush zone ruptured during escalation of the eruption vigour during phase 3. The LSR and its crystals are inferred to be closely similar in their characteristics to the feedstock magma that generated the melt-dominant body and evolved through subsequent cooling and fractionation to form the HSR. In overall terms, the evidence from the crystal phases demonstrates that a super-sized rhyolite magma body can be physically created in a geologically very short period of time. The compositional textures and data for all the mineral phases, both previously published and newly presented in this work, yield a consistent story of extraordinarily rapid extraction of LSR melt and entrained crystals into a rapidly evolving and cooling HSR body. When coupled with field constraints these data establish a central role for extensional tectonics in regulating the pre-and syn-eruptive processes and their timings in the Oruanui system.</p>

The Oruanui Eruption: Insights into the Generation and Dynamics of the World's Youngest Supereruption

<p>This work investigates the pre- and syn-eruptive magmatic processes that culminated in the world’s youngest supereruption – the ~25.4 ka, 530 km³ Oruanui eruption from Taupo volcano, New Zealand – from the perspective of crystals contained in single parcels of frozen magma (pumice). The eruption is unusual in its variety of magmatic compositions. About 98-99 % by mass of the juvenile material is high-SiO₂ rhyolite (HSR; >74 wt% SiO₂), with lesser volumes of tholeiitic and calc-alkaline mafic magmas (total 3-5 km³; basaltic andesite to andesite: 53-63 % SiO₂), low-silica rhyolite (LSR: 0.1-0.5 km³; <74 wt% SiO₂) and a ‘foreign’ biotite-bearing rhyolite from an adjacent magma source (0.03 km³; ~74 wt% SiO₂). Detailed textural and chemical data from amphibole, plagioclase, and orthopyroxene are placed within the context of an established time-stratigraphic, volcanological and petrographic framework, of unrivalled detail globally for an eruption of this age and magnitude. Other previously published information from zircon and quartz is also incorporated. This unique contextual information is used to constrain observations and inferences regarding the processes that moved the Oruanui magma from a largely uneruptible crystal-rich progenitor at depth (where an eruption was possible), to a highly eruptible melt-rich magma at shallow crustal levels (where eruption was inevitable). A thermally and compositionally stratified crystal mush body, with an upper SiO₂-saturated and quartz bearing cap at ~3.5 km depth and quartz-free roots extended down to at least ~10 km. This inference is made on three bases. 1) That the quartz cores contain trapped melt that is more evolved than the melt component of the immediately pre-eruptive magma body, indicating their growth within mush from a more evolved interstitial melt. 2) The majority of plagioclase, amphibole, and orthopyroxene cores, in contrast to quartz have compositions that indicate growth from less evolved melts than that encountered in the final melt-dominant magma body. 3) Barometric estimates from amphibole core compositions indicate derivation from a range of depths (~3.5 to 10 km). The spatial and temporal transitions from mush to melt-dominant magma body are recorded in the textural and compositional zonations within the crystal phases. Crystals from all levels of the zoned mush body were entrained during the melt extraction process resulting in a diversity of crystal compositions being brought together in the melt-dominant magma body. Textural disequilibrium features in the cores of orthopyroxene and plagioclase crystals reflect their temporary departure from stability during the accompanying significant decompression (recorded in the amphibole model pressures). Counterpart chemical signatures, reflecting this partial orthopyroxene and plagioclase dissolution, are recorded in the amphiboles which show no textural evidence for destabilisation during ascent. Crystal chemical and textural zonation in the rim growths of the plagioclase, orthopyroxene, and amphibole record further crystallisation in the accumulating melt-dominant magma body, and reflect cooling and compositional evolution of the body towards its final pre-eruptive conditions. The timing of growth of the melt dominant magma body is constrained by Fe-Mg diffusion modelling of key boundaries in orthopyroxene crystals. Accumulation of this body began only ~1600 years and peaked at 230 years prior to the eruption, as vast volumes of melt and entrained crystals were drained from the mush body and began to accumulate at shallower levels (~3.5 to 6.0 km depth). Within the thin, sill-like melt-dominant magma body, significant heat loss drove vigorous convection. Textural and chemical zonation patterns within the rim-zones of plagioclase, orthopyroxene and amphibole, inferred to have grown solely in the melt-dominant magma body, depict a secular cooling and melt evolution trends towards final uniform thermal (~770 °C) and compositional conditions inferred for the HSR magma. Despite the rapid accumulation of a vast volume of crystal-poor HSR magma at shallow crustal levels, the apparent gas-saturated nature of that magma, and vigorous convection within the melt-dominant magma body itself, the chronologies from HSR orthopyroxene imply that the magma underwent a period of stasis of about 60 years. The presence of 3-16 wt% of ‘foreign’ biotite-bearing juvenile pumices in the early Oruanui fall deposits (phases 1 and 2) show that coincident with the onset of the Oruanui eruption, magma was transported laterally in a dike from an adjacent independent magma system 10-15 km to the NNE to intersect the active Oruanui conduit. Consideration of the tectonic stress orientations associated with this lateral transport imply that an external tectonic influence through a major rifting event was a critical factor in the initiation of the Oruanui eruption. Only the presence of the foreign magma, and linkages to detailed field-based and geochemical constraints enables the tectonic influence to be identified. During the eruption itself, minor quantities of Oruanui LSR magma were erupted , and with a crystal cargo, reflecting derivation from deeper (mostly >6 km), hotter (~820 °C) sources in the crystal mush roots to the system. Comparisons of LSR crystal compositions with cores to many HSR crystals for plagioclase, orthopyroxene and amphibole imply that the LSR magma was derived from pockets in the mush zone ruptured during escalation of the eruption vigour during phase 3. The LSR and its crystals are inferred to be closely similar in their characteristics to the feedstock magma that generated the melt-dominant body and evolved through subsequent cooling and fractionation to form the HSR. In overall terms, the evidence from the crystal phases demonstrates that a super-sized rhyolite magma body can be physically created in a geologically very short period of time. The compositional textures and data for all the mineral phases, both previously published and newly presented in this work, yield a consistent story of extraordinarily rapid extraction of LSR melt and entrained crystals into a rapidly evolving and cooling HSR body. When coupled with field constraints these data establish a central role for extensional tectonics in regulating the pre-and syn-eruptive processes and their timings in the Oruanui system.</p>

Age and Geochemistry of Late Jurassic Mafic Volcanic Rocks in the Northwestern Erguna Block, Northeast China

The northwestern Erguna Block, where a wide range of volcanic rocks are present, provides one of the foremost locations to investigate Mesozoic Paleo-Pacific and Mongol-Okhotsk subduction. The identification and study of Late Jurassic mafic volcanic rocks in the Badaguan area of northwestern Erguna is of particular significance for the investigation of volcanic magma sources and their compositional evolution. Detailed petrological, geochemical, and zircon U-Pb dating suggests that the Late Jurassic mafic volcanic rocks formed at 157–161 Ma. Furthermore, the geochemical signatures of these mafic volcanic rocks indicate that they are calc-alkaline or transitional series with weak peraluminous characteristics. The rocks have a strong MgO, Al2O3, and total alkali content, and a SiO2 content of 53.55–63.68 wt %; they are enriched in Rb, Th, U, K, and light rare-earth elements (LREE), and depleted in high-field-strength elements (HFSE), similar to igneous rocks in subduction zones. These characteristics indicate that the Late Jurassic mafic volcanic rocks in the Badaguan area may be derived from the partial melting of the lithospheric mantle as it was metasomatized by subduction-related fluid and the possible incorporation of some subducting sediments. Subsequently, the fractional crystallization of Fe and Ti oxides occurred during magmatic evolution. Combined with the regional geological data, it is inferred that the studied mafic volcanic rocks were formed by lithospheric extension after the closure of the Mongol-Okhotsk Ocean.

System-level fractionation of carbon from disk and planetesimal processing

&lt;div class=&quot;page&quot; title=&quot;Page 1&quot;&gt; &lt;div class=&quot;section&quot;&gt; &lt;div class=&quot;layoutArea&quot;&gt; &lt;div class=&quot;column&quot;&gt; &lt;p&gt;Finding and characterizing extrasolar Earth analogs will rely on interpretation of the planetary system&amp;#8217;s environmental context. The total budget and fractionation between C&amp;#8211;H&amp;#8211;O species sensitively affect the climatic and geodynamic state of terrestrial worlds, but their main delivery channels are poorly constrained. We connect numerical models of volatile chemistry and pebble coagulation in the circumstellar disk with the internal compositional evolution of planetesimals during the primary accretion phase. Our simulations demonstrate that disk chemistry and degassing from planetesimals operate on comparable timescales and can fractionate the relative abundances of major water and carbon carriers by orders of magnitude. As a result, individual planetary systems with significant planetesimal processing display increased correlation in the volatile budget of planetary building blocks relative to no internal heating. Planetesimal processing in a subset of systems increases the variance of volatile contents across planetary systems. Our simulations thus suggest that exoplanetary atmospheric compositions may provide constraints on &lt;em&gt;when&lt;/em&gt; a specific planet formed.&lt;/p&gt; &lt;/div&gt; &lt;/div&gt; &lt;/div&gt; &lt;/div&gt;