Magma Extraction Research Articles

Volcano-pluton connections at the Lake City magmatic center (Colorado, USA)

Abstract Exposed at the Lake City caldera (Colorado, USA) is the ca. 23 Ma reversely stratified (rhyolite to trachyte) Sunshine Peak Tuff and post-collapse syenite and monzonite resurgent intrusions. Existing models for this system suggest that the rhyolites are related to the trachyte and resurgent syenite through fractional crystallization, separation, and remobilization (crystal mush model), and that multiple magma batches were involved in the system (Hon, 1987; Kennedy et al., 2016; Lubbers et al., 2020). We use U-Pb zircon CA-ID-TIMS-TEA and zircon trace-element modeling to further probe age and geochemical relationships between the extrusive and intrusive units. Zircon ages and compositions from the erupted units and the syenite overlap, suggesting these magmas were related and may have mixed prior to eruption. Results from the monzonite suggest it was a contemporaneous but distinct magma batch that mixed with parts of the larger system. Trends in zircon geochemistry are decoupled from time, reflecting a complex history of accessory mineral saturation and mixing of magma batches, and a distinct high-Hf population of zircon grains hints at the existence of an additional, independent batch of rhyolitic magma in the system. The new ages we present shorten the lifetime of the Lake City magmatic system from 80 to 300 k.y. (Bove et al., 2001) to 60 to 220 k.y. and suggest the high-silica rhyolite magma crystallized over a minimum of ~160 k.y. This latter timescale likely reflects a protracted history that includes differentiation of a parent melt prior to extraction of eruptible high-silica rhyolite magma.

Experimental Investigation of Trachydacite Magma Storage Prior to the 1257 Eruption of Mt Samalas

Abstract The caldera-forming Samalas eruption of the Samalas–Rinjani volcanic complex on Lombok, Indonesia, in 1257 CE ranks as one of the most explosive and sulphur-rich eruptions of the past thousand years. Along with other significant events (e.g. Tambora 1815, El Chichón 1982), Samalas 1257 forms a class of powerful eruptions of magmas of intermediate alkaline composition, which remain relatively understudied. This study aims to better understand the pre-eruptive magma reservoir conditions and volatile storage capacities of intermediate alkaline systems, using the 1257 Samalas eruption as a case study. Systematic partial-equilibrium experiments were run to constrain potential magma reservoir conditions (25–200 MPa and 850–1000°C) by examining changes in the chemistry and textures of experimental glass and mineral products. Natural trachydacite pumice from the 1257 Samalas eruption was used as the experimental charge for a series of water-saturated experiments run in cold-seal pressure vessels, buffered at an oxygen fugacity ƒO2 of NNO/NNO + 1 log units. Assessing the match between natural and experimental glass compositions and mineral phases and textures reveals that pre-eruptive magma reservoir conditions were between 875°C and 930°C and ca. 100–150 MPa (4.5 ± 1km depth). Breakdown of experimental amphibole at pressures below 75 MPa, and plagioclase instability at 950°C/100 MPa, provide the strongest constraints on phase stabilities that are consistent with the magma storage region. In the observed temperature and pressure range, the natural compositions of plagioclase, amphibole and orthopyroxene are replicated in experimentally precipitated rims. Clinopyroxene and high anorthite plagioclase (An>64) in the natural mineral assemblage could not be replicated in the experiments, implying that these compositions most likely did not grow in the shallow pre-eruptive reservoir of Mt Samalas. The pre-eruptive storage depths of trachydacite magma are significantly shallower than the postulated magma storage at the currently active basaltic-andesite magmatic system at Rinjani, and thus monitoring magma storage depth may be a useful indicator of changing magma composition in the future. Our findings show that highly explosive VEI 7 eruptions of intermediate alkaline magma can be fed from a relatively limited range of storage pressures (100–150 MPa), suggesting accumulation of magma in one place, rather than the rapid extraction of magma from a vertically extensive, transcrustal magma system.

Pegmatites as geological expressions of spontaneous crustal flow localisation

Amongst the silicate-rich crystalline rocks that are produced in the continental crust, pegmatites are characterised by their large crystals which give them both an aesthetic and economic interest. Pegmatites crystallise either from fractionated magma derived from a parent granitic body or from the partial melting of meta-sediments or meta-igneous rocks (e.g. amphibolite). The mechanism of residual magma (or fluid) extraction from the parent granitic body has been thoroughly studied, but pegmatitic melt extraction after partial melting has received less attention. We present here a series of non-dimensional numerical experiments using a two-phase flow formulation that couples the Stokes problem to/with non-linear Darcy flow. This approach makes it possible to predict the movement of fluid inclusions (named porosity) in a deformable of a viscous rocks (named porous matrix). We find that the simulation produces either clusters or an isolated body of fluid inclusion depending on the compaction/decompaction ratio of the effectively viscous matrix in which they rise. Using a review of pegmatite natural properties, we propose a scaling of our numerical simulations that describes the ascent of a pegmatite-forming melt produced by partial melting. We then discuss possible travel distances and temperature effects. To discuss our results in light of field observations, we assume that the compaction-decompaction ratio is an accurate proxy for the influence of brittle processes at a scale smaller than the representative volume element, and therefore corresponds structural level variations at which pegmatites are emplaced. We find that our numerical simulation explain the statistical organisation, in terms of level of emplacement, of real fields of pegmatites possibly derived from partial melting of meta-sediments. Pegmatites in fact tend to organise as clusters around brittle faults in upper crustal levels, whereas they present a scattered distribution at mid to lower crustal levels. Our results therefore show that porosity waves are a possible mechanism for rapidly extracting and transporting pegmatite melts formed during low-degree (ca. 10%) partial melting at distances up to a few kilometres in the crust.

Formation of green-core clinopyroxene in continental basalts through magmatic differentiation and crustal assimilation: Insights from in-situ trace element and Pb isotopic compositions

Green-core clinopyroxene (GCPX) widely occurs in diverse magmas and various tectonic settings. Due to its special textual and compositional zonation, it is usually considered as solid evidence for involvement of exotic felsic melt and some unique geological processes (e.g., magma mixing, mantle metasomatism and recycling of foundered lower continental crust (LCC)). However, this premise is challenged by its more complicated and enigmatic origins than previously thought. To explore potential origins of GCPXs in continental basalts, here we investigate in-situ trace element and Pb isotopic compositions of the GCPXs in the Feixian and Sihetun primitive basalts from the eastern North China Craton (NCC). The results show that the Feixian GCPXs can be subdivided into three types in addition to the Sihetun GCPXs. Regardless of the types of these GCPXs, their surrounding rims are regarded as overgrowths that crystallized from the basaltic host magmas. The wide range of Mg# (67–86), trace element patterns similar to those of clinopyroxene equilibrated with the basaltic host magmas and host lava-like Pb isotopic ratios of the Feixian type-1 GCPXs suggest that they have crystallized from differentiated melts genetically related to the basaltic host magmas. Besides unradiogenic Pb isotopic ratios, the Feixian type-2, type-3 and Sihetun GCPXs are characterized trace element signatures that are identical or similar to those of clinopyroxenes in pyroxene-rich granulite, garnet-bearing granulite and feldspar-rich granulite xenoliths from the NCC, respectively, indicating their derivation from lower crustal granulite wall rocks during the ascending of the basaltic host magmas. Our work suggests that GCPXs can be simply formed through the two common magmatic processes of continental basaltic magmas (i.e., crystallization differentiation and crustal assimilation) and highlights that exotic felsic melt and unique geological processes are not additionally required despite being possible. Before using GCPXs as an indicator of specific magmatic processes, comprehensive data (e.g., trace element and isotopic compositions) should be carefully investigated for their exact origins, especially for those in continental magmas. Additionally, GCPXs may provide valuable information about the structure and composition of the lower crust and a window to unravel crustal assimilation and digestion during magma extraction and their influence on compositions of basaltic magmas.

Magma chamber decompression during explosive caldera-forming eruption of Aira caldera

Decompression of a magma chamber is a fundamental condition of caldera collapse. Although theoretical models have predicted the decompression of magma chambers before caldera collapse, few previous studies have demonstrated the amount of magma chamber decompression. Here, we determine water content in quartz glass embayments and inclusions from pyroclastic deposits of a caldera-forming eruption at Aira volcano approximately 30,000 years ago and apply this data to calculate decompression inside the magma chamber. We identify a pressure drop from 140–260 MPa to 20–90 MPa during the extraction of around 50 km3 of magma prior to the caldera collapse. The magma extraction may have caused down-sag subsidence at the caldera center before the onset of catastrophic caldera collapse. We propose that this deformation resulted in the fracturing and collapse of the roof rock into the magma chamber, leading to the eruption of massive ignimbrite.

Dynamics of Episodic Magma Injection and Migration at Yellowstone Caldera: Revisiting the 2004–2009 Episode of Caldera Uplift With InSAR and GPS Data

AbstractThe 2004–2009 caldera uplift is the largest instrumentally recorded episode of unrest at Yellowstone caldera. We use GPS and Interferometric Synthetic Aperture Radar (InSAR) time series spanning 2004–2015, with a focus in the aforementioned event to understand the mechanisms of unrest. InSAR data recorded ∼25 and ∼20 cm of uplift at the Sour Creek (SCD) and Mallard Lake (MLD) resurgent domes during 2004–2009, and ∼8 cm of subsidence at the Norris Geyser Basin (NGB) during 2004–2008. The SCD/MLD uplift was followed by subsidence across the caldera floor with a maximum at MLD of ∼1.5–2.5 cm/yr, and no deformation at NGB. The best‐fit source models for the 2004–2009 period are two horizontal sills at depths of ∼8.7 and 10.6 km for the caldera source and NGB, respectively, with volume changes of 0.354 and −0.121 km3, and an overpressure of ∼0.1 MPa. The InSAR and GPS time series record exponentially increasing followed by exponentially decreasing uplift between 2004 and 2009, which is indicative of magma injection into the caldera reservoir, with no need for other mechanisms of unrest. However, magma extraction from NGB to the caldera is unable to explain the subsidence coeval with the caldera uplift. Models of magma injection can also explain other episodes of caldera uplift like that in 2014–2015. Distributed sill opening models show that magma is stored across the caldera source with no clear boundary between MLD and SCD. Since the magma overpressure is orders of magnitude below the tensile strength of the encasing rock, historical episodes of unrest like these are very unlikely to trigger an eruption.

The magma source of small-scale intraplate monogenetic volcanic systems in northern New Zealand

The Auckland intraplate volcanic province consists of four small fields of basaltic monogenetic volcanoes. Each of these was active for a period of ~1 Myr and during the last 3 Myr the locus of magmatic activity migrated northward in discrete ~50 km steps. The most recent of these volcano fields is the Auckland Volcanic Field (AVF) in which individual eruptions have produced discrete magma batches within the composition range nephelinite to sub-alkaline basalt. This range of compositions is explained by partial melting involving variable melt/solid ratios together with modal heterogeneity in their mantle source. Eruptions in the field have been irregularly spaced during its 200 kyr life with repose periods varying between ≤0.1 and 13 kyr. Although most of Auckland's volcanoes represent a single period of eruption, paired eruption sequences in which smaller more alkaline magma batches have been followed by more voluminous less alkalic magmas have also occurred. A significant flare-up in activity at about 30 kyr produced compositionally discrete eruptions from at least five spatially separate locations all within a temporal interval of as little as 100 years. An interpretation of these data is that each event (volcano) in the AVF represents a discrete batch of magma extracted from an source with the capacity of producing a compositional variety of magma. Such a source forms when upwelling mantle undergoes adiabatic rise and exists as a metastable entity at the asthenosphere-lithosphere boundary for prolonged periods (105–106 years) during which time it is tapped to create individual magma batches. The shorter term magma extraction events that produced individual volcanoes in the AVF are suggested to be the result of crustal scale tectonic events. The behaviour of these four spatially separate monogenetic volcanic systems at temporal intervals of <106 years is likely linked to the long term tectonic evolution of the Pacific-Australian Plate boundary beneath the New Zealand region.

What Triggers Caldera Ring‐Fault Subsidence at Ambrym Volcano? Insights From the 2015 Dike Intrusion and Eruption

AbstractSurface deformation accompanying dike intrusions is dominated by uplift and horizontal motion directly related to the intrusions. In some cases, it includes subsidence due to associated magma reservoir deflation. When reservoir deflation is large enough, it can form, or reactivate preexisting, caldera ring‐faults. Ring‐fault reactivation, however, is rarely observed during moderate‐sized eruptions. On February 21, 2015 at Ambrym volcano in Vanuatu, a basaltic dike intrusion produced more than 1 m of coeruptive uplift, as measured by InSAR, synthetic aperture radar correlation, and Multiple Aperture Interferometry. Here, we show that an average of ∼40 cm of slip occurred on a normal caldera ring‐fault during this moderate‐sized (VEI &lt; 3) event, which intruded a volume of ∼24 × 106 m3 and erupted ∼9.3 × 106 m3 of lava (DRE). Using the 3D Mixed Boundary Element Method, we explore the stress change imposed by the opening dike and the depressurizing reservoir on a passive, frictionless fault. Normal fault slip is promoted when stress is transferred from a depressurizing reservoir beneath one of Ambrym's main craters. After estimating magma compressibility, we provide an upper bound on the critical fraction (f = 7%) of magma extracted from the reservoir to trigger fault slip. We infer that broad basaltic calderas may form in part by hundreds of subsidence episodes no greater than a few meters, as a result of magma extraction from the reservoir during moderate‐sized dike intrusions.

Repetitive Duality of Rhyolite Compositions, Timescales, and Storage and Extraction Conditions for Pleistocene Caldera-forming Eruptions, Hokkaido, Japan

Abstract During the Early Pleistocene, numerous caldera-forming eruptions occurred in the southernmost Kurile arc (central Hokkaido, Japan), building an extensive pyroclastic plateau with an area &amp;gt;1600 km2. The arc remains active today, and proximity to populations and infrastructure makes understanding these magmatic systems a critical endeavor. We investigate three major caldera-forming ignimbrite eruptions: Biei (c. 2·0 Ma), Tokachi (c. 1·2 Ma), and Tokachi–Mitsumata (c. 1·0 Ma), with an emphasis on constraining the pressures of magma extraction and storage and the timescales of crystallization. Although all pumice glass compositions from the three eruptions are high-silica rhyolites (77–78 wt% SiO2), hierarchical clustering analysis of major and trace element glass data indicates that the Tokachi and Tokachi–Mistumata ignimbrites each have two distinct pumice populations (Type-1F and Type-2F). We find that these two distinct pumice types record pre-eruptive temperatures, extraction pressures, and crystallization timescales that are strikingly similar between the two eruptions. Using the rhyolite-MELTS geobarometer, we estimate that although all magma types from all three eruptions had storage pressures of 50–150 MPa (∼2–6 km), Type-1F magma was extracted from a deeper mush reservoir (200–450 MPa) compared with Type-2F (100–200 MPa). Pre-eruptive temperatures, constrained by plagioclase–liquid equilibration thermometry and rhyolite-MELTS, suggest that Type-1F magma in both eruptions was hotter (800–820 °C) compared with Type-2F (780–800 °C), but that both reached thermal equilibrium upon eruption (760–780 °C). Because zircon is observed only as inclusions and rarely in contact with glass, we conclude that all magmas were zircon-undersaturated, and thus zircon saturation temperatures, which are 60–100 °C lower than those estimated by the other three independent thermometers, underestimate magmatic temperatures. Using these temperatures as minimum estimates, diffusional relaxation times of Ti zonation in quartz, as revealed by cathodoluminescence (CL), give absolute maximum quartz residence times of &amp;lt;1800 years for Type-2F samples and &amp;lt;600 years for Type-1F for all samples; residence times are &amp;lt;300 years for all samples if the more reasonable Fe–Ti oxide temperature is used instead (∼770 °C). Our modelling therefore suggests that the melt-dominated rhyolite magmas that fed these caldera-forming eruptions were ephemeral features that crystallized within the shallow crust for centuries to several millennia. Rapid rim growth occurred in all magma types in all three eruptions, with a majority of quartz rims (10–200 µm) having grown in less than 2 years. Using plagioclase textures and major and trace element data, we conclude that the bright-CL rims of quartz resulted from decompression and subsequent rapid growth, rather than by a recharge-driven heating event. Thus, decompression occurred within 2 years prior to eruption. Remarkably, the two distinct magma types are statistically similar in terms of composition, crystallization timescales, magma storage conditions, and extraction depths, despite being from eruptions that occurred 240 kyr apart, and from calderas that are separated by 35 km. This suggests magma assembly and storage processes that are spatiotemporally repetitive in this region of Hokkaido.

The Effects of Degassing on Magmatic Gas Waves and Long Period Eruptive Precursors at Silicic Volcanoes

AbstractCyclical ground deformation, associated seismicity, and elevated degassing are important precursors to explosive eruptions at silicic volcanoes. Regular intervals for elevated activity (6–30 hr) have been observed at volcanoes such as Mount Pinatubo in the Philippines and Soufrière Hills in Montserrat. Here, we explore a hypothesis originally proposed by Michaut et al. (2013, https://doi.org/10.1038/ngeo1928) where porosity waves containing magmatic gas are responsible for the observed periodic behavior. We use two‐phase theory to construct a model where volatile‐rich, bubbly, viscous magma rises and decompresses. We conduct numerical experiments where magma gas waves with various frequencies are imposed at the base of the model volcanic conduit. We numerically verify the results of Michaut et al. (2013, https://doi.org/10.1038/ngeo1928) and then expand on the model by allowing magma viscosity to vary as a function of dissolved water and crystal content. Numerical experiments show that gas exsolution tends to damp the growth of porosity waves during decompression. The instability and resultant growth or decay of gas wave amplitude depends strongly on the gas density gradient and the ratio of the characteristic magma extraction rate to the characteristic magma degassing rate (Damköhler number, Da). We find that slow degassing can lead to a previously unrecognized filtering effect, where low‐frequency gas waves may grow in amplitude. These waves may set the periodicity of the eruptive precursors, such as those observed at Soufrière Hills Volcano. We demonstrate that degassed, crystal‐rich magma is susceptible to the growth of gas waves which may result in the periodic behavior.

The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption

SUMMARYOn 10 May 2018, an unprecedented long and intense seismic crisis started offshore, east of Mayotte, the easternmost of the Comoros volcanic islands. The population felt hundreds of events. Over the course of 1 yr, 32 earthquakes with magnitude greater than 5 occurred, including the largest event ever recorded in the Comoros (Mw = 5.9 on 15 May 2018). Earthquakes are clustered in space and time. Unusual intense long lasting monochromatic very long period events were also registered. From early July 2018, Global Navigation Satellite System (GNSS) stations and Interferometric Synthetic Aperture Radar (InSAR) registered a large drift, testimony of a large offshore deflation. We describe the onset and the evolution of a large magmatic event thanks to the analysis of the seismicity from the initiation of the crisis through its first year, compared to the ground deformation observation (GNSS and InSAR) and modelling. We discriminate and characterize the initial fracturing phase, the phase of magma intrusion and dyke propagation from depth to the subsurface, and the eruptive phase that starts on 3 July 2018, around 50 d after the first seismic events. The eruption is not terminated 2 yr after its initiation, with the persistence of an unusual seismicity, whose pattern has been similar since summer 2018, including episodic very low frequency events presenting a harmonic oscillation with a period of ∼16 s. From July 2018, the whole Mayotte Island drifted eastward and downward at a slightly increasing rate until reaching a peak in late 2018. At the apex, the mean deformation rate was 224 mm yr−1 eastward and 186 mm yr−1 downward. During 2019, the deformation smoothly decreased and in January 2020, it was less than 20 per cent of its peak value. A deflation model of a magma reservoir buried in a homogenous half space fits well the data. The modelled reservoir is located 45 ± 5 km east of Mayotte, at a depth of 28 ± 3 km and the inferred magma extraction at the apex was ∼94 m3 s−1. The introduction of a small secondary source located beneath Mayotte Island at the same depth as the main one improves the fit by 20 per cent. While the rate of the main source drops by a factor of 5 during 2019, the rate of the secondary source remains stable. This might be a clue of the occurrence of relaxation at depth that may continue for some time after the end of the eruption. According to our model, the total volume extracted from the deep reservoir was ∼2.65 km3 in January 2020. This is the largest offshore volcanic event ever quantitatively documented. This seismo-volcanic crisis is consistent with the trans-tensional regime along Comoros archipelago.

Accumulation of rhyolite magma and triggers for a caldera-forming eruption of the Aira Caldera, Japan

The sudden destabilization of voluminous magma after stable accumulation in a crustal magma chamber is a key process in the sequence of a catastrophic caldera-forming eruption. We investigated the petrological characteristics of magma that has erupted before, during and after the caldera-forming eruption of the Aira Caldera, Japan. This provides an example of the evolution of a rhyolite magma system during a catastrophic caldera-forming eruption. Stratigraphic and petrological investigations on the pre-caldera eruptive materials showed accumulation of high-silica rhyolite 4–5 km below the ground surface, which commenced at least ~ 30,000 years prior to the caldera-forming eruption. A part of the accumulated rhyolite magma leaked from the chamber and caused several pre-caldera eruptions. Phase equilibrium relationships and water concentrations in glass inclusions in phenocrysts suggest that the vertical extension of rhyolite magma chamber was 2–2.5 km. The rhyolite magma maintained a temperature between 735 and 800 °C and crystal mush state with 20–50% of crystallinity during its accumulation. The injection of mafic magmas destabilized the stored magma. Further, the heating of the rhyolite magma to 780–840 °C by the heat transfer from the mafic magma decreased the crystallinity down to 10% and induced mobilization of the stored rhyolite magmas. The remobilized magma could erupt as the Plinian eruption at the beginning and produced the Osumi pumice-fall deposit. This deposit occupies approximately 10% of the total volume of the erupted magma during the caldera-forming eruption. Though the extraction of rhyolite magma was limited to part of the magma chamber, decompression of the magma chamber by the magma extraction was enough to induce the caldera collapse and resulted in the eruption of the main ignimbrite (Ito ignimbrite) from the rest of the magma chamber. The volume of the erupted magma during the caldera-forming eruption is comparable with that of the magma chamber. New rhyolite magma with discernible composition from the rhyolite magmas of the caldera-forming eruption started accumulating in the magma chamber following the caldera-forming eruption and was the source for the post-caldera eruptions.

The Fate of Accessory Minerals and Key Trace Elements During Anatexis and Magma Extraction

AbstractGranite genesis and crustal evolution are closely associated with partial melting in the lower or middle crust and extraction of granite magmas to upper crustal levels. This is generally thought to be the leading mechanism by which the upper continental crust became enriched in incompatible components such as the heat-producing elements U and Th through time. However, field evidence from anatectic terrains, the source rocks of granite magmas, raises doubt about the efficiency of this process. Leucosomes and associated leucogranites, representative of melts in such terrains, are often depleted in U, Th and REE compared to their source and therefore unable to enrich the upper crust in these elements. This paper demonstrates using anatectic turbidites exposed on Kangaroo Island that accessory minerals, the main hosts of U, Th and REE, become preferentially concentrated in the melanosomes, effectively removing these elements from the melt. Whole rock geochemistry and detailed petrography suggests that (1) peraluminous melts dissolve only small fractions of monazite and xenotime, because efficient apatite dissolution saturates melt early in phosphorous; and (2) local melt–host reaction emerging from melt migration may cause melt to crystallize in the magma extraction channelways in or close to the magma source region. Crystallization causes oversaturation of the magma triggering crystallization and capture of accessory minerals in the growing biotite-rich selvedge rather than in the melt channel itself. Crystallization of accessory minerals away from the leucosome explains the apparent under-saturation of elements hosted by these accessory minerals in the leucosome and leucogranites. While intense reworking of thick piles of turbidites, common in accretionary orogens, reflect important processes of crustal formation, the fate of accessory phases and the key elements they control, such as the heat producing elements U and Th, are strongly dependent on the interaction between melt and surrounding solids during segregation and extraction.

Neoarchean Synmagmatic Crustal Extrusion in the Transpressional Yilgarn Orogen

AbstractPositive feedback mechanisms between magmatism and transpression allow efficient extraction, transfer, and delivery of synorogenic granitic magma. Although many Archean orogens are dominantly transpressional and record voluminous production of synorogenic granites, the role of crustal magmatism in Archean tectonics has been so far little explored. We detail here the structural evolution of the Ballard shear zone, a major synorogenic, inclined transpressional shear zone from the upper‐crustal granite‐greenstone system of the Neoarchean Yilgarn Orogen (Western Australia). We show that the bulk of the three‐dimensional deformation was efficiently partitioned between greenstones and crystallizing granitic sheets, with the latter accommodating large amounts of orogen‐parallel strain through hypersolidus viscous flow. Other large synorogenic Yilgarn shear zones show a similar structural evolution, allowing the formulation of a general tectonomagmatic model. The positive feedback between protracted magmatism and transpression promoted lower‐ to middle‐crustal extraction and upward transfer of syntectonic magma. In upper‐crustal sink regions, the rheological dichotomy between cold and strong greenstones and hot and weak sheets of crystallizing granite allowed efficient kinematic partitioning, resulting in the extrusion of the partially molten shear zone cores. This sequence of events occurred repeatedly throughout the orogeny, following magma pulses, with the bulk of the shearing along each shear zone waning with the cooling of each syntectonic pluton. Synorogenic magmatism played therefore a major role in promoting and localizing transpressional deformation, and ultimately in shaping the architecture of the whole Yilgarn orogen.

Origin of late Neoarchean granitoid diversity in the Western Shandong province, North China Craton

Diverse late Neoarchean granitoids are widely exposed in the Western Shandong province (WSP). According to geological relationships, lithological features, magmatic zircon U–Pb–Lu–Hf isotopes and whole-rock chemical characteristics, eight main lithological assemblages are identified, and their petrogenesis is determined. (1) The ~2.54 Ga and ~2.51 Ga potassic granites show the highest SiO2 contents (67.06–75.84 wt%) and were sourced from partial melting of metamorphic greywackes. (2) The ~2.53 Ga quartz syenites exhibit the highest alkali contents (10.32–11.23 wt%) and were sourced from mantle-derived basaltic rocks and recycled sediments. (3) The ~2.53–2.52 Ga quartz diorites (less-differentiated sanukitoids) formed from partial melting of metasomatized mantle. (4) The ~2.53 Ga hornblendites have crystallization ages and Lu–Hf isotopes similar to those of the quartz diorites, implying that they are cogenetic cumulates from the quartz dioritic magmas. (5) The ~2.56–2.52 Ga tonalite-trondhjemite-granodiorite (TTG) gneisses were formed by melting of thickened crust. (6) The ~2.53 Ga high-Mg granodiorites (differentiated sanukitoids) were formed by mixing of crust-mantle melts. (7) The ~2.51–2.49 Ga gabbros originated from partial melting of fluid-metasomatized mantle. (8) The ~2.54–2.53 Ga high-Mg andesites (boninites) originated from partial melting of refractory mantle that experienced the extraction of basaltic magma at earlier stage and was then enriched in LILEs and LREEs by slab-derived melts.Integrating the petrogenesis of these late Neoarchean diversified granitoids and associated volcanic rocks, we consider that the late Neoarchean was a crucial period of crust-mantle interaction in the WSP, and partial melting of the metasomatized mantle produced intermediate-mafic melts, then underplating of the mantle-derived magmas caused the partial melting of crustal materials, further led to the mixing of crust-mantle magmas. The above complex crust-mantle interactions triggered the transfer of energy and materials from the Earth’s interior to the surface, forming modern-like continental crust thickness and geothermal gradient. Moreover, combining late Neoarchean intense granitic magmatism and ~2.49 Ga mafic dike swarms implies that the North China Craton experienced cratonization processes at the end of the Neoarchean.

Volcanic–plutonic connection and associated Au‐Cu mineralization of the Tulasu ore district, Western Tianshan, NW China: Implications for mineralization potential in Palaeozoic arc terranes

Constructing the plutonic–volcanic connections in volcanic arc setting is essential for understanding a variety of magmatic processes from continental crust differentiation to formation of magmatic‐hydrothermal ore deposits. Based on the combination of zircon U–Pb geochronology and Hf isotope and whole‐rock geochemistry and Sr‐Nd isotopes, we evaluated the potential connections between volcanic and plutonic rocks from the Tulasu Basin, the largest epithermal gold ore district in the Chinese Western Tianshan. The peak of volcanism coincides well with the major plutonism, representing an important magma event at ~347–367 Ma. These contemporaneous volcanic and plutonic rocks show broadly similar arc‐related geochemical signatures and zircon in‐situ Hf (εHf[t] = −1.0–14.3) and whole‐rock Nd (εNd[t] = −4.2–4.6) isotopic compositions, indicative of formation from parental magmas derived from a common depleted mantle source metasomatized by slab‐derived melts, despite diverse consequent processes (fractional crystallization, crustal contamination, and magma mixing/mingling) during their differentiation in a subduction‐related setting. Volcanic rocks from the Axi caldera in the centre of the Tulasu district are geochemically less evolved than the coeval porphyritic granite, which indicates that the intrusive rocks might originate from residual melts left behind by extraction of erupted magmas. In contrast, the porphyry Cu mineralized Kexiaxi pluton, which occurs in the margin of the Tulasu district, is compositionally indistinguishable from the coeval volcanic suites. This implies that the Kexiaxi pluton represents the intrusive equivalent of a common magma reservoir feeding the volcanic eruptions, and magma fluxes may have controlled their differentiation. We thus suggest that the depleted mantle derived calc‐alkaline magma, modified during slab‐related melt metasomatism, crustal contamination, and magma mixing may have played an important role in copper enrichment at Kexiaxi. Finally, the relatively water‐rich and fertile nature of arc magma of the Tulasu district indicates high exploration potential for significant porphyry Cu‐Au deposits. The existing alteration‐immobile fertility proxy (V/Sc) for intrusive rocks may also be useful to assess the fertility of cogenetic volcanic rocks, in turn with implications for unexposed porphyry Cu deposits at depths.

Petrogenesis and geodynamic significance of Kayinde gabbro in the Ashele Basin, Altay Orogenic Belt, Xinjiang, Northwest China: Constraints from geochronological and geochemical data

Mafic‐ultramafic intrusions occur sporadically but are pivotal to constrain the tectonic evolution of the Chinese Altay, a key part of the Central Asian Orogenic Belt. The tectonic setting of the Early‐Middle Devonian is well documented, but that of the Late Devonian remains a matter of ongoing debate. The Kayinde gabbro occurs in the Ashele Basin of the South Altay. This paper reports petrologic, zircon U‐Pb geochronological, geochemical, and Sr‐Nd isotopic data of the Kayinde gabbro, with the aims to investigate its formation time, petrogenesis, and geodynamic significance. Zircons separated from the gabbro yield a cluster of concordant data points and a weighted mean age of 373.1 ± 4.1 Ma, representing a Late Devonian emplacement. Geochemically, the Kayinde gabbro is subalkaline, tholeiitic, and characterized by high Mg# values (68–73), enrichment in large‐ion lithophile elements (including Ba, Sr, Rb, and U), depletion in high‐field‐strength elements (including Nb, Ta, Zr, and Hf), fairly flat REE patterns, low (87Sr/86Sr)i (0.70366–0.70440) and positive εNd(t) (3.4–5.4) values. The geochemical characteristics collectively suggest that the Kayinde gabbro was derived from a spinel‐stable lithospheric mantle source, which was depleted by previous magma extraction and metasomatized by later slab‐derived fluids or melts in an island arc setting. The geochemical similarities between the Kayinde gabbro and other contemporary mafic‐ultramafic intrusions reveal that the subduction continued during the Late Devonian in the Chinese Altay. In combination with the published research, we favour a two‐unit arc system composed of a continental arc and an island arc, spanning across the Early to Late Devonian. The Ashele Basin can be considered to be an oceanward setting in this model.

Unified quantitative observation of coexisting volcanic sulfur dioxide and sulfate aerosols using ground-based Fourier transform infrared spectroscopy

Abstract. We developed an optimal-estimation algorithm to simultaneously retrieve, for the first time, coexisting volcanic gaseous SO2 and sulfate aerosols (SA) from ground-based Fourier transform infrared (FTIR) observations. These effluents, both linked to magmatic degassing process and subsequent atmospheric evolution processes, have overlapping spectral signatures leading to mutual potential interferences when retrieving one species without considering the other. We show that significant overestimations may be introduced in SO2 retrievals if the radiative impact of coexistent SA is not accounted for, which may have impacted existing SO2 long-term series, e.g. from satellite platforms. The method was applied to proximal observations at Masaya volcano, where SO2 and SA concentrations, and SA acidity, were retrieved. A gas-to-particle sulfur partitioning of 400 and a strong SA acidity (sulfuric acid concentration: 65 %) were found, consistent with past in situ observations at this volcano. This method is easily exportable to other volcanoes to monitor magma extraction processes and the atmospheric sulfur cycle in the case of ash-free plumes.

Ubiquitous ultra-depleted domains in Earth’s mantle

Partial melting of Earth’s mantle generates oceanic crust and leaves behind a chemically depleted residual mantle. The time-integrated composition of this chemically depleted mantle is generally inferred from basalts produced at mid-ocean ridges. However, isotopic differences between oceanic mantle rocks and mid-ocean ridge basalts suggest that mantle and basalt composition could differ. Here we measure neodymium isotope ratios in olivine-hosted melt inclusions from lavas of the Azores mantle plume. We find neodymium isotope ratios that include the highest values measured in basalts, and suggest that melts from ultra-depleted mantle contribute to the isotopic diversity of the erupted lavas. Ultra-depleted melts have exceedingly low preservation potential during magma extraction and evolution due to progressive mixing with melts that are enriched in incompatible elements. A notable contribution of ultra-depleted melts to the Azores mantle plume therefore implies that variably depleted mantle is the volumetrically dominant component of the Azores plume. We argue that variably depleted mantle, sometimes ranging to ultra-depleted compositions, may be a ubiquitous part of most ocean island and mid-ocean ridge basalt sources. If so, Earth’s mantle may be more depleted than previously thought, which has important implications for the rate of mass exchange between crust and mantle, plume dynamics and compositional stratification of Earth’s mantle. Depleted mantle is a volumetrically dominant component of the Azores plume and possibly of oceanic basalt sources more generally, according to neodymium isotope compositions of olivine-hosted melt inclusions from lavas of the Azores mantle plume.

Magma extraction pressures and the architecture of volcanic plumbing systems

The deposits of large volcanic eruptions provide the sole record of the architecture of magmatic plumbing systems in the moments when large pools of crystal-poor, eruptible magma are present in the crust. It is widely accepted that silicic magmas form by segregation of melt-rich, crystal-poor magma from a crystal-rich source; however, the depths at which segregation takes place and the distribution of the magma within the crust are not well constrained. We present a new approach to calculate pressures at which crystal-poor, eruptible magma is extracted from a crystal-rich source (i.e. mush). We apply the approach to a sequence of large (>50 km3) eruptions from the Taupo Volcanic Zone (TVZ), New Zealand, which were part of a volcanic flare-up. We compare the calculated extraction pressures with pre-eruptive storage pressures for the same units. Our results show that storage and extraction pressures do not always coincide. Instead, eruptible magma can be completely segregated from the crystal-rich source, and stored at shallower levels in the crust prior to eruption. In the case of the TVZ flare-up, repeated input of material and heat – probably coupled with tectonic extension – gradually conditioned the crust and allowed extraction of eruptible magma over a growing range of pressures with time. Our approach has the potential to reveal important information on the structure and distribution of magmatic systems within the shallow crust.