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New geochemical data, including Sr–Nd–Pb isotopes for whole-rock and groundmass samples, are reported for edifice-forming eruptives at Tongariro volcano, New Zealand, which span its ~ 350 ka to late Holocene history. Tongariro eruptives are medium-K basaltic-andesites to dacites (53.0–66.2 wt% SiO2) that evolved via assimilation-fractional crystallisation (AFC) processes partly or mostly in the uppermost 15 km of the crust. When ordered chronologically using a high-resolution 40Ar/39Ar-dated eruptive stratigraphy, the compositional data show systematic 10–130 kyr cycles. Mafic replenishment events inferred from MgO values occurred at ~ 230, ~ 151, ~ 88 and ~ 56 ka and in the late Holocene, with high-MgO flank vents erupting at ~ 160, ~ 117, ~ 35 and ~ 17.5 ka. Cycles in Sm/Nd, 87Sr/86Sr, 143Nd/144Nd and Pb isotopic ratios, which are decoupled from MgO, K2O and Rb/Sr cycles, indicate periods of prolonged crustal residence of magmas from ~ 230 to ~ 100 ka and ~ 95 to ~ 30 ka. AFC modelling shows that intermediate and silicic melt compositions, with r-values between 0.1 and 1, are needed to reproduce Tongariro compositional arrays. AFC models also indicate that ~ 20% of the average Tongariro magma comprises assimilated (meta)sedimentary basement material. Locally, Tongariro and adjacent Ruapehu volcanoes attain their most crust-like 87Sr/86Sr and 143Nd/144Nd compositions at ~ 100 and ~ 30 ka, paralleling with zircon model-age crystallisation modes at the rhyolitic Taupō volcano ~ 50 km to the NNE. These coincidences suggest that the timing and tempo of magma assembly processes at all three volcanoes were contemporaneous and may have been tectonically influenced since at least 200 ka. [ABSTRACT FROM AUTHOR]

Volatile measurements in mineral-hosted sealed melt inclusions, and open-ended embayments, have previously been used to study magma ascent dynamics in large rhyolitic eruptions. However, despite occurring more frequently, smaller-volume explosive events remain under-studied. We present magmatic volatile data from quartz-hosted melt inclusions and embayments for eight post-25.4 ka rhyolitic eruptions at Ōkataina Volcanic Centre, Aotearoa New Zealand. Seven originated from within the main caldera, and the other erupted from the associated Ōkareka Structural Embayment. Melt inclusions preserve volatile contents of 2.92–5.82 wt% H2O and 13–126 ppm CO2, indicating pre-eruptive storage depths of 4.5–7.4 km, with younger eruptions being more shallow. The lack of correlation between H2O, CO2, inclusion size or distance to the crystal rim suggests magma bodies experienced variable degrees of degassing during magma storage, with some amount of post-entrapment volatile modification prior to and concurrent with final magma ascent. Diffusion modelling of measured H2O gradients in melt embayments indicates ascent rates of 0.10–1.67 m.s−1 over time spans of 20–230 min for the intra-caldera events. In contrast, ascent rates for the eruption from the Ōkareka Structural Embayment may be more rapid, at 1.59–4.4 m.s−1 over a time span of 22–34 min. Our findings imply that the final, pre-eruptive magma movement towards the surface could be less than a few hours. Comparisons with published data for caldera-forming explosive events reveal no clear relationships between final ascent rate, eruption size or initial volatile content, implying that other factors besides eruption volume control rhyolite magma ascent. [ABSTRACT FROM AUTHOR]

Oxygen isotopes are useful for tracing interactions between magmas, crustal rocks and surface-derived waters. We use them here to consider links between voluminous silicic magmatism and large-scale hydrothermal circulation in New Zealand's central Taupō Volcanic Zone (TVZ). We present >350 measurements of plagioclase, quartz, hornblende and groundmass glass δ18O values from 40 eruptions from three discrete magmatic systems: Ōkataina and Taupō calderas, and the smaller Northeast Dome system. For each mineral, mean δ18O values vary by ~1% (δ18Oplag = +6.7-7.8%, δ18Oqtz = +7.7-+8.7%, δ18Ohbl = +5.4-+6.4%, δ18Oglass = +7.1-+8.0%), and inter-mineral fractionations mostly reflect high-temperature equilibria. Outliers (e.g., ~+6% or >+10% plagioclase) represent contaminants incorporated on short-enough timescales to preserve disequilibrium (~102 yrs for plagioclase). Melt δ18O values calculated from phenocrysts are ~+7.3-+8.0%. Where multiple magmas were involved in the same eruption their δ18Omelt values are indistinguishable, implying that their parental mushes were isotopically well-mixed. However, small (≤0.5%) but consistent δ18Omelt value gradients occur over millennial timescales at Ōkataina and Taupō, with short-term ~0.4-0.5% decreases in δ18Omelt values over successive post-caldera eruptions correlating with increases in 87Sr/86Sr. These changes reflect tens of percent assimilation of a mixture of hydrothermally altered silicic plutonic material and higher-87Sr/86Sr greywacke. These examples represent the first evidence for assimilation of altered crust into TVZ magmas. The subtle and short-lived isotopic signals of these interactions are only recognized through the high temporal resolution of the TVZ eruptive record and complementary radiogenic isotope data. Similar interactions may have been obscured in other nominally highor normal-δ18O magmatic systems. [ABSTRACT FROM AUTHOR]

Ilmenite-, magnetite- and clinopyroxene−melt trace element partition coefficients were investigated experimentally as a function of oxygen fugacity and melt composition in a range of synthetic ferrobasaltic bulk compositions. The experiments were performed at a constant temperature (1080 °C) and pressure (1 atm) over a range of oxygen fugacity (fO2) conditions from ca. 2 log units below to ca. 2 log units above the FMQ buffer. The partitioning behaviour of the divalent cations Zn, Mn, Co and Ni are found to be controlled by the degree of polymerisation of the coexisting melt; the partitioning behaviour of rare earth elements, Y and Sc can be explained well by the lattice strain model and the partitioning of the high-field strength elements Zr, Hf, Ta and Nb is influenced by the TiO2 content of the melt. Vanadium partitioning is strongly influenced by oxygen fugacity and a series of linear regression equations are presented to express the dependence of the mineral−melt partitioning behaviour of the multivalent cation V on oxygen fugacity. Furthermore, calibration of the partitioning of vanadium between magnetite–ilmenite pairs as an oxybarometer is proposed and applied to a ferrobasaltic layered intrusion—the Skaergaard intrusion—to provide an estimate of oxygen fugacity. [ABSTRACT FROM AUTHOR]

We consider here the validity, accuracy, and precision of rhyolite-MELTS modelling in inferring the pre-eruptive magma storage conditions for the caldera-forming 25.5 ka Oruanui and 232 CE (1.72 ka) Taupō eruptions at Taupō volcano, New Zealand as proposed by Pamukçu et al. (2020: Contrib Mineral Petrol 175: 48). There are four major issues with the modelling as used. First, the model is inappropriately applied for the Taupō event as this magma does not contain phenocrystic quartz. Second, the products of both eruptions, as is typical for virtually all Taupō Volcanic Zone rhyolites, contain plagioclase but lack sanidine, leading to increased model errors beyond those stated. Third, temperatures utilised for modelling crystallisation histories for both magmas are in conflict with independent measures of magmatic temperatures for these rocks. Fourth, glass compositions for each of these eruptions individually fall within analytical uncertainty yet yield model pressures that vary over hundreds of MPa, in part due to contrasting analytical protocols. We conclude that rhyolite-MELTS is too imprecise (errors of ± ~ 100 MPa or more for the methods applied) for quartz + 1 feldspar systems like at Taupō volcano specifically and the Taupō Volcanic Zone in general to be applied with confidence to deriving relative or absolute depths of magma storage. [ABSTRACT FROM AUTHOR]

Taupō volcano (New Zealand) is distinguished as the source of Earth's youngest supereruption (∼25.5 ka), with Lake Taupō occupying the resulting caldera. Taupō has also produced eruptions of a wide variety of sizes, styles and associated landscape responses over a ∼350 kyr period. Early Taupō (>54 ka) is poorly demarcated, merging with Maroa to the north, and is represented by widely scattered, geochemically distinct, effusive domes and explosive eruption products from vents all around the modern lake. Taupō had two independent magmatic systems from 54–25.5 ka, one that led to the Oruanui event focussed beneath the area of the modern lake and a second, northeast of the lake that has remained active to the present. Following the Oruanui supereruption, the rebuilt modern hyperactive Taupō magmatic system is primarily focussed beneath the lake and has generated 25 rhyolitic eruptions since ∼12 ka. The young rhyolite magmas come from an evolving silicic magma reservoir, but vary widely in their eruptive sizes and destructive potential. In the modern era Taupō experiences unrest every decade or so, but uncertainties remain over the nature of the magma reservoir and the processes that drive unrest or eruptive activity that require new geophysical data and interpretations. [ABSTRACT FROM AUTHOR]

New zircon U–Th model-age and trace element datasets are presented from Taupo volcano (New Zealand), which are used to investigate the timescales and broad-scale magmatic processes involving zircon crystallization after the caldera-forming 25·4 ka Oruanui supereruption. Detailed 14C-based chronologies and controls on vent locations allow the timing and location of post-caldera eruptions to be spatially and temporally constrained to an extent not possible for any other supervolcano. After ∼5 kyr of post-Oruanui quiescence, Taupo erupted three dacitic units, followed by another ∼5 kyr break, and then a sequence of rhyolitic units in three subgroups (SG1–SG3) from 12 ka onwards. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U–Th zircon model ages in Taupo SG1 rhyolites (erupted from 12 to 10 ka) indicate only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains in some units. U–Pb dating of these older grains shows that both 300–450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units that were vented within the outline of the Whakamaru caldera. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolite (Units B and C) have notably high concentrations of U, Th, P, Y + (REE)3+ and Nb but with only minor variations in Hf and Ti. SG2 zircons typically have higher Sc contents, reflecting large-scale changes in melt chemistry and crystallizing mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U–Th ages from single post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallization, acting within a crustal protolith chemically independent of that which was dominant in the Oruanui system. These periods of cooling and crystallization alternate with times of rejuvenation and eruption, sometimes demonstrably accompanying syn-eruptive regional rifting and mafic magma injection. Not only were the processes that developed the supersized Oruanui magma body rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale. [ABSTRACT FROM AUTHOR]

The 2·08 Ma, ∼2500 km3 Huckleberry Ridge Tuff (HRT) eruption, Yellowstone, generated two fall deposits and three ignimbrite members (A, B, C), accompanying a ∼95 x 65 km caldera collapse. Field data imply that the pre-A fall deposits took weeks to be erupted, then breaks of weeks to months occurred between members A and B, and years to decades between B and C. We present compositional and isotopic data from single silicic clasts (pumice or fiamme) in the three ignimbrite members, plus new data from co-eruptive mafic components to reconstruct the nature and evacuation history of the HRT crustal magmatic complex. Geochemical data, building on field characteristics, are used to group nine silicic clast types into seven compositional suites (A1-A3; B1; C1-C3) within their respective members A, B and C. Isotopic data are then added to define four magmatic systems that were tapped simultaneously and/or sequentially during the eruption. Systems 1 and 2 fed the initial fall deposits and then vented throughout member A, accompanied by trace amounts of mafic magma. In member A, volumetrically dominant system 1 is represented by a rhyolite suite (A1: 73·0–77·7 wt % SiO2, 450–1680 ppm Ba) plus a distinct low-silica rhyolite suite (A2: 69·2–71·6 wt % SiO2, >2500 ppm Ba). System 2 yielded only a low-Ba, high-silica rhyolite suite (A3: 76·7–77·4 wt % SiO2, ≤250 ppm Ba). Glass compositions in pumices from systems 1 and 2 show clustering, indicative of the same multiple melt-dominant bodies identified in the initial fall deposits and earliest ignimbrite. Member B samples define suite B1 (70·7–77·4 wt % SiO2, 540–3040 ppm Ba) derived from magmatic system 1 (but not 2) that had undergone mixing and reorganisation during the A: B time break, accompanying mafic magma inputs. Mafic scoriae erupted in upper member B cover similar compositions to the member A clasts, but extend over a much broader compositional range. Member C clast compositions reflect major changes during the B: C time break, including rejuvenation of magmatic system 2 (last seen in member A) as suite C3 (75·3–77·2 wt % SiO2, 100–410 ppm Ba), plus the appearance of two new suites with strong crustal signatures. Suite C2 is another rhyolite (74·7–77·6 wt % SiO2, with Ba decreasing with silica from 2840 to 470 ppm) that defines magmatic system 3. Suite C2 also shows clustered glass compositions, suggesting that multiple melt-dominant bodies were a repetitive feature of the HRT magmatic complex. Suite C1, in contrast, is dacite to rhyolite (65·6–75·0 wt % SiO2, with Ba increasing with silica from 750 to 1710 ppm) that defines magmatic system 4. Compositions from magmatic systems 1 and 2 dominantly reflect fractional crystallization, but include partial melting of cumulates related to earlier intrusions of the same mafic magmas as those syn-eruptively vented. Country rock assimilation was limited to minor amounts of a more radiogenic (with respect to Sr) evolved contaminant. In contrast, systems 3 and 4 show similar strongly crustal isotopic compositions (despite their differences in elemental composition) consistent with assimilation of Archean rocks via partial melts derived from cumulates associated with contrasting mafic lineages. System 3 links to the same HRT mafic compositions co-erupted in members A and B. In contrast, system 4 links to olivine tholeiite compositions erupted in the Yellowstone area before, sparsely during, and following the HRT itself. All four magmatic systems were housed beneath the HRT caldera area. Systems 1 and 2 were hosted in Archean crust that had been modified by Cretaceous/Eocene magmatism, whereas systems 3 and 4 were hosted within crust that retained Archean isotopic characteristics. The extreme compositional diversity in the HRT highlights the spatial and temporal complexities that can be associated with large-volume silicic magmatism. [ABSTRACT FROM AUTHOR]

Cumulate rocks of the Upper Main Zone and Upper Zone (UUMZ) of the Bushveld Complex, South Africa, contain the world's major resources of Fe-Ti-V6P, hosted in Ti-magnetite and apatite, and are commonly considered as having crystallized from the last major injection of magma into the magma chamber. In this study, we present the petrography, modal proportions, whole-rock major element chemistry (260 samples), electron microprobe data (~10 000 analyses for plagioclase, olivine, and pyroxene), and compiled analyses of Cr in magnetite (239 samples) for the UUMZ sampled over 2·1 kmof the Bierkraal drill cores in the western limb of the Complex. The UUMZ section exhibits a broad normal fractionation trend upwards, but a series of reversals to more primitive anorthite contents in plagioclase, Mg# in pyroxenes and olivine, Cr in whole-rocks and Cr in magnetite separates are observed, accompanied by the appearance or disappearance of various minerals. Anorthosite or leucogabbro layers are closely linked to these reversals; the reversals in An % of plagioclase are used as boundaries to divide the UUMZ into 18 cycles. These cycles are interpreted as indications of magma chamber replenishment by plagioclase-laden magmas (up to 20 vol. % plagioclase) and are also marked by spikes in Cr content. In addition, abundant Fe-Ti oxide-bearing plagioclase-rich rocks are identified in the lower half of the UUMZ. These have crystallized from a hybrid melt produced by the mixing of a new plagioclase-bearing magma batch and the resident magma. Further crystallization of this hybrid liquid may lead to the formation of magnetite layers in the lower part of the UUMZ. The Bushveld UUMZ therefore grew by multiple emplacements of crystal-laden magmas coming from deep-seated chambers. Slow cooling in a shallow chamber explains the systematic bottom-up compositional evolution in the cumulate pile within individual cycles. The residual melt reached silicate liquid immiscibility soon after the saturation of apatite. Thereafter, segregation of conjugate Fe-rich and Si-rich melts and crystallization of the paired melts produces cumulates with a smooth upward decrease in Fe-Ti oxides, whereas plagioclase mode increases in each apatite-bearing cycle. A comparison of systematic geochemical analyses and a detailed lithological stratigraphy between the Bushveld limbs demonstrates the possible connectivity between the western and eastern Upper Zone but indicates notable differences from the Bellevue section of the northern limb. [ABSTRACT FROM AUTHOR]

The silicic (broadly dacitic to rhyolitic) magmatic systems that feed supereruptions show great diversity, but have in common a role for mafic (broadly basaltic to andesitic) magmas as drivers of the systems. Here we document the mafic component in the rhyolitic magmatic system of the 2.08 Ma Huckleberry Ridge Tuff (HRT), Yellowstone, and compare it to mafic materials erupted prior to and following the HRT eruption in the area within and immediately around its associated caldera. The HRT eruption generated initial fall deposits, then three ignimbrite members A, B, and C, with further fall deposits locally separating B and C. A “scoria” component was previously known from the upper B ignimbrite, but we additionally recognize juvenile mafic material as a sparse component in early A, locally abundant in upper A and sparsely in lower B. It has not been found in the C ignimbrite. In upper B the mafic material is vesicular, black to oxidized red-brown scoria, but at other sites is overwhelmingly non-vesicular, and sparsely porphyritic to aphyric. Despite their contrasting appearances and occurrences, the mafic components form a coherent compositional suite from 49.3–63.3 wt% SiO2, with high alkalis (Na2O+K2O = 4.5–7.3 wt%), high P2O5 (0.52–1.80 wt%), and notably high concentrations of both high field strength and large-ion lithophile elements (e.g., Zr = 790–1830 ppm; Ba = 2650–3800 ppm). Coupled with the trace-element data, Sr-Nd-Pb isotopic systematics show influences from Archean age lower crust and lithospheric mantle modified by metasomatism during the late Cretaceous to Eocene, as previously proposed for extensive Eocene magmatism/volcanism around the Yellowstone area. The HRT mafic compositions contrast markedly with the Snake River Plain olivine tholeiites erupted before and after the HRT eruption, but are broadly similar in several respects to the generally small-volume Craters of the Moon-type mafic to intermediate lavas erupted recently just west of the HRT caldera, as well as farther west in their type area. The combination of trace element and isotopic data on the HRT mafics are only consistent with an origin for their parental magma as melts from mantle enriched by high temperature and pressure melts, most likely from the underlying Farallon slab. Subsequent interaction of the HRT mafic magmas occurred with the Archean lower crust and lithospheric mantle, but not the highly radiogenic upper crust in this area. The close temporal and spatial relationships of the HRT mafic compositions and the preceding Snake River Plain olivine tholeiite eruptives suggest a high degree of spatial heterogeneity in the mantle beneath the Yellowstone area during the early (and subsequent) development of its modern magmatic system. [ABSTRACT FROM AUTHOR]

The caldera-forming 26·5 ka Oruanui eruption (Taupo, New Zealand) erupted ∼530 km3 of magma, >99% rhyolitic, <1% mafic. The rhyolite varies from 71·8 to 76·7 wt % SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt % SiO2. Average rhyolite compositions at each stratigraphic level do not change significantly through the eruption sequence. Oxide geothermometry, phase equilibria and volatile contents imply magma storage at 830–760°C, and 100–200 MPa. Most rhyolite compositional variations are explicable by ∼28% crystal fractionation involving the phenocryst and accessory phases (plagioclase, orthopyroxene, hornblende, quartz, magnetite, ilmenite, apatite and zircon). However, scatter in some element concentrations and 87Sr/86Sr ratios, and the presence of non-equilibrium crystal compositions imply that mixing of liquids, phenocrysts and inherited crystals was also important in assembling the compositional spectrum of rhyolite. Mafic compositions comprise a tholeiitic group (52·3–63·3 wt % SiO2) formed by fractionation and crustal contamination of a contaminated tholeiitic basalt, and a calc-alkaline group (56·7–60·5 wt % SiO2) formed by mixing of a primitive olivine–plagioclase basalt with rhyolitic and tholeiitic mafic magmas. Both mafic groups are distinct from other Taupo Volcanic Zone eruptives of comparable SiO2 content. Development and destruction by eruption of the Oruanui magma body occurred within ∼40 kyr and Oruanui compositions have not been replicated in vigorous younger activity. The Oruanui rhyolite did not form in a single stage of evolution from a more primitive forerunner but by rapid rejuvenation of a longer-lived polygenetic, multi-age ‘stockpile’ of silicic plutonic components in the Taupo magmatic system. [ABSTRACT FROM AUTHOR]

Bulk analyses of plagioclase and high-alumina orthopyroxene megacrysts (decimetre- to metre-sized crystals with plagioclase lamellae) from the late Proterozoic Egersund–Ogna massif-type anorthosite (Rogaland anorthosite province, SW Norway) are used to constrain the parental magma compositions and differentiation processes within this 30 km diameter diapir. Spatial compositional variations show that two types of anorthosite occur: high-Sr (720–1090 ppm) andesine anorthosite (An48 ± 4 ) in the centre of the intrusion and low-Sr (320–620 ppm) labradorite anorthosite (An59 ± 6 ) in the margin. Two populations of orthopyroxene megacrysts are clearly discriminated by their Mn and Cr contents, but display similar ranges of Mg-number (55–79). Interpretation of trace element data and comparison with inversion models suggest that the two types of anorthosite result from crystallization of different parental magma compositions with high-alumina basalt affinities and similar Mg-numbers, but different wollastonite, Sr, Mn and Cr contents. A Rayleigh fractional crystallization model of the trace element concentrations vs MgO content in orthopyroxene constrains the cotectic orthopyroxene:plagioclase proportions to 0·33:0·67 in both magma types before the appearance of Fe–Ti oxide minerals on the liquidus at F ∼0·7 (where F is melt fraction). Polybaric crystallization is recorded by variable alumina contents in the orthopyroxene megacrysts (2·3–8·5 wt %), corresponding to crystallization pressures in the 3–13 kbar range as shown by experimental data. This implies that the high-alumina orthopyroxene megacrysts mainly crystallized en route during diapiric rise of the anorthositic mush. Modelling the plagioclase compositions with experimentally determined partition coefficients for Ca, K, Sr and Ba confirms that pressure variation during polybaric crystallization is the main controlling factor on compositional variations in the anorthosite pluton. [ABSTRACT FROM AUTHOR]

The Sept Iles layered intrusion (Quebec, Canada; 564 Ma) is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro, and an upper border series with anorthosite, capped by cupolas of A-type granite. Chilled margin compositions suggest a ferrobasaltic parental magma close to that of the Skaergaard intrusion, but much richer in iron and titanium. Samples from drill-cores and surface sampling of the 4·7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 Fe–Ti oxide layers cm- to m-thick and many anorthositic autolithic blocks. The sequence of crystallization in the layered series is: plagioclase (An72–34) and olivine (Fo75–21) followed by magnetite and ilmenite, then Ca-rich pyroxene and finally apatite. An olivine gap is observed between Fo66 and Fo59. The saturation of Fe–Ti oxides before Ca-rich pyroxene is interpreted to be the result of the high FeOt and TiO2 contents and the low CaO content of the parental magma. Contamination by old continental crust has occurred during crystallization of the layered series, as indicated by Sr isotopic compositions (87Sr/86Sr564 = 0·70360–0·70497). The differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases, Cr content of magnetite and lower Sr isotope ratios. These reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma. [ABSTRACT FROM PUBLISHER]

Isotopic fingerprinting has long been used to trace magmatic processes and the components that contribute to magmas. Recent technological improvements have provided an opportunity to analyze isotopic compositions on the scale of individual crystals, and consequently to integrate isotopic and geochemical tracing with textural and petrographic observations. It has now become clear that mineral phases are commonly not in isotopic equilibrium with their host glass/groundmass. Isotopic ratios recorded from core to rim of a mineral grain reflect the progressive changes in the magma composition from which the mineral crystallized. The sense of these changes and the relationship between isotopic composition and petrographic features, such as dissolution surfaces, can be used to constrain magma evolution pathways involving open system processes such as magma mixing, contamination and recharge. [ABSTRACT FROM AUTHOR]

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The formation of Fe-Ti oxides-rich layers is commonly associated with open-system magma chamber dynamics. These processes are widely discussed due to the economic importance of Fe-Ti-(V) deposits, although an alkaline-system approach to the matter is still scarce. In this study, we use petrography, mineral chemistry, X-ray diffraction and elemental geochemical analyses (whole-rock and Sr isotopes) to discuss the process associated with the formation of Fe-Ti-(V) oxide-rich clinopyroxenite (OCP, 7–15 vol.%) and magnetitite (MTT, 85 vol.%) from the Ponte Nova alkaline mafic–ultramafic massif (PN, K-Ar 87.6 Ma). Ilmenite and Ti-magnetite from both OCP and MTT exhibit higher MgO contents (MgO > 5.0 wt%) than other PN rocks. OCP shows high 87Sr/86Sri ratios, equivalent to crustal-contaminated lithotypes of the PN Central Intrusion, while MTTs are less radiogenic. The oxide supersaturation in silicate mafic magmas is typically associated with the dislocation of the liquid cotectic evolution line, shifting to Fe-Ti-(V) oxide minerals stability field, mainly Ti-magnetite. Different magmatic processes can lead to these changes such as crustal contamination and magma recharge. For the PN massif, the OCP was formed by the assimilation of crustal contaminants in a mush region, near the magma chamber upper walls, which was associated with the evolution of the main pulse. Differently, the MTT would have its origin related to the interaction between magma chamber evolved liquids and more primitive liquids during a new episode of magma recharge. Lastly, post-magmatic events were superimposed on these rocks, generating sulfides. [ABSTRACT FROM AUTHOR]

Magma ascent and eruption are driven by a set of internally and externally generated stresses that act upon the magma. We present microstructural maps around melt inclusions in quartz crystals from six large rhyolitic eruptions using synchrotron Laue X-ray microdiffraction to quantify elastic residual strain and stress. We measure plastic strain using average diffraction peak width and lattice misorientation, highlighting dislocations and subgrain boundaries. Quartz crystals across studied magma systems preserve similar and relatively small magnitudes of elastic residual stress (mean 53–135 MPa, median 46–116 MPa) in comparison to the strength of quartz (~ 10 GPa). However, the distribution of strain in the lattice around inclusions varies between samples. We hypothesize that dislocation and twin systems may be established during compaction of crystal-rich magma, which affects the magnitude and distribution of preserved elastic strains. Given the lack of stress-free haloes around faceted inclusions, we conclude that most residual strain and stress was imparted after inclusion faceting. Fragmentation may be one of the final strain events that superimposes stresses of ~ 100 MPa across all studied crystals. Overall, volcanic quartz crystals preserve complex, overprinted deformation textures indicating that quartz crystals have prolonged deformation histories throughout storage, fragmentation, and eruption. [ABSTRACT FROM AUTHOR]

During their formation, terrestrial planets underwent a magma ocean phase during which their metallic cores segregated from their silicate mantles and their early atmospheres formed. These planetary formation processes resulted in a redistribution of the abundances of highly volatile elements (HVEs, such as H, C, N, and S) between the planets' metallic cores, silicate mantles, and atmospheres. This review presents the numerous experimental techniques used to simulate the conditions and identify the parameters that influenced the behavior of HVEs during planetary formation. We also review the analytical techniques used to characterize the different types of experimental samples and quantify the distribution of HVEs between metallic and silicate phases, as well as their solubilities in silicate glasses. This exhaustive review targets students and young researchers beginning their work on the subject, or, more generally, scientists seeking a better understanding of this field of research. [ABSTRACT FROM AUTHOR]

Determining the time spans of processes related to the assembly of eruptible magma at active volcanoes is fundamental to understand magma chamber dynamics and assess volcanic hazard. This information can be recorded in the chemical zoning of crystals. Nevertheless, this kind of study is still poorly employed for the active volcanoes of the Neapolitan area (Southern Italy), in particular, for Ischia island where the risk is extremely high and this information can provide the basis for probabilistic volcanic hazard assessment. For these reasons, we acquired chemical composition on clinopyroxene crystals erupted at Ischia during the Zaro eruption (6.6 ± 2.2 ka) and performed numerical simulations of the input of mafic magma into a trachytic reservoir, in order to investigate various aspects of pre-eruptive dynamics occurring at different timescales. This event emplaced a ~ 0.1 km3 lava complex, in which the main trachytic lava flows host abundant mafic to felsic enclaves. Previous petrological investigation suggested that mafic magma(s) mixed/mingled with a trachytic one, before the eruption. In this work, the clinopyroxene zoning patterns depict the growth of crystals in different magmatic environments, recording sequential changes occurred in the plumbing system before the eruption. The evolution of the plumbing system involved a hierarchy of timescales: a few hours for magma mingling caused by mafic recharge(s) and likely occurred multiple times over a decade during which a dominant magmatic environment was sustained before the eruption. Such timescales must be considered in volcanic hazard assessment at Ischia and similar active volcanoes in densely populated areas. [ABSTRACT FROM AUTHOR]

Modeling the behavior of trace elements during lunar magma ocean solidification is important to further our understanding of the chemical evolution of the Moon. Lunar magma ocean evolution models rely on consistent datasets on how trace elements partition between a lunar silicate melt and coexisting minerals at different pressures, temperatures, and redox conditions. Here we report new experimental trace element partition coefficients (D) between clinopyroxene (cpx), pigeonite, orthopyroxene, plagioclase, olivine (ol), and silicate melt at conditions relevant for the lunar magma ocean. The data include Dcpx−melt at ambient and high pressures (1.5 GPa and 1310 °C), and partition coefficients at ambient pressure for pig, opx, ol, and pl. Overall, clinopyroxene is a phase that may control the fractionation of key geochemical trace element ratios, such as Lu/Hf and Sm/Nd, during the evolution of the lunar magma ocean. We explore the impact of the new silicate Dmineral−melt on the trace element evolution of the lunar magma ocean and we find that accessory phosphate minerals, such as apatite or whitlockite are of critical importance to explain the observed trace element and isotopic signature of the KREEP reservoir on the Moon. The new partition coefficients were applied to calculate the trace element evolution of the residual melts of the crystallizing lunar magma ocean and we propose a new trace element composition for the urKREEP reservoir. The new data will be useful for future thermo-chemical models in order to adequately predict the duration of the lunar magma ocean and the age of the Moon. [ABSTRACT FROM AUTHOR]

Processes taking place within the magma plumbing system can exert an important control on the composition of mid-ocean ridge basalts (MORB). Plagioclase ultraphyric basalts (PUBs) found at magma-poor mid-ocean ridges exhibit diverse disequilibrium characteristics, which can provide vital insights for distinguishing the complex effects of melt transport from those of source heterogeneity on the compositions of MORBs. Here, we present new insights into magmatic processes using integrated petrologic and geochemical studies of the PUBs from two zones (~ 50° and ~ 64°E longitude) along the ultraslow-spreading southwest Indian ridge (SWIR). The studied PUBs have complex mineral morphologies, including skeletal and acicular crystals, glomerocrysts with open and closed structure, reverse and normally zoned crystals and external and internal resorption even in single samples. Both low- and high-Fo olivine and An plagioclase crystals are in disequilibrium with their matrix glasses. Some plagioclase phenocrysts have repeated oscillatory zoning (An77–86) going from their core to rim and an abrupt decrease in An content toward the rim. Disequilibrium Sr isotopic compositions are present at several scales: between cores and rims of plagioclase crystals, between different plagioclase crystals and between plagioclase and their host lavas. Inferred pressures of magma storage range from 0.3 to 11.3 kbar. The textural and compositional diversity of crystals together with the variability in melt compositions reflect the combined influences of source heterogeneity and magmatic processes (e.g. crystallization, assimilation and magma mixing processes) taking place within crystal mushes. Our data combined with previous studies suggest that the magmatic processes within the SWIR magma plumbing system involve formation, disaggregation and juxtaposition of crystal-rich mush zones. [ABSTRACT FROM AUTHOR]

The Messejana–Plasencia great dyke (MPGD) is a Late Triassic tholeiitic gabbro intrusion related to the Central Atlantic Magmatic Province. Its large outcrop extent (~530 km), combined with its prolongation below the Duero basin (additional 100 km), makes it one of the world's largest dykes known. We have studied felsic granophyric bodies appearing in its northernmost segment at different scales, from mm-sized (interstitial micrographic pockets) to felsic dykes of up to 10 m thick and 1.5 km long, intruding within the gabbros. Significant differences exist in the mineral and whole-rock composition of gabbros and granophyres, including the Sr–Nd isotopic ratios. The chemical variation in the gabbros is coherent with fractionation of olivine, clinopyroxene and plagioclase at depth. However, the presence of a compositional gap between gabbros and granophyres (absence of intermediate compositions) and the formation of these late-stage intergranular felsic melts within the gabbro mesostasis suggest that they could be derived by liquid immiscibility. The Sr–Nd isotopic heterogeneity in the MPGD gabbros and the presence of zircons with Variscan ages (~286 Ma), inherited from granulitic rocks, indicate that the mafic magmas experienced some degree of lower crust assimilation during fractionation close to the Moho depth. On the contrary, the scarce xenocrystic Variscan zircon crystals found in a granophyric dyke within the MPGD gabbro display similar textures and ages (~299 Ma) to those of the country rock granites and point to contamination at a different crustal level. [ABSTRACT FROM AUTHOR]

Batholith-sized bodies of crystal-rich magmatic ‘mush’ are widely inferred to represent the hidden sources of many large-volume high-silica rhyolite eruptive units. Occasionally these mush bodies are ejected along with their trapped interstitial liquid, forming the distinctive crystal-rich ignimbrites known as ‘monotonous intermediates’. These ignimbrites are notable for their combination of high crystal contents (35–55%), dacitic bulk compositions with interstitial high-silica rhyolitic glass, and general lack of compositional zonation. The 5000 km3 Fish Canyon Tuff is an archetypal eruption deposit of this type, and is the largest known silicic eruption on Earth. Ejecta from the Fish Canyon magmatic system are notable for the limited compositional variation that they define on the basis of whole-rock chemistry, whereas ∼ 45 vol. % crystals in a matrix of high-silica rhyolite glass together span a large range of mineral-scale isotopic variability (microns to millimetres). Rb/Sr isotopic analyses of single crystals (sanidine, plagioclase, biotite, hornblende, apatite, titanite) and sampling by micromilling of selected zones within glass plus sanidine and plagioclase crystals document widespread isotopic disequilibrium at many scales. High and variable 87Sr/86Sri values for euhedral biotite grains cannot be explained by any model involving closed-system radiogenic ingrowth, and they are difficult to rationalize unless much of this radiogenic Sr has been introduced at a late stage via assimilation of local Proterozoic crust. Hornblende is the only phase that approaches isotopic equilibrium with the surrounding melt, but the melt (glass) was isotopically heterogeneous at the millimetre scale, and was therefore apparently contaminated with radiogenic Sr shortly prior to eruption. The other mineral phases (plagioclase, sanidine, titanite, and apatite) have significantly lower 87Sr/86Sri values than whole-rock values (as much as −0·0005). Such isotopic disequilibrium implies that feldspars, titanite and apatite are antecrysts that crystallized from less radiogenic melt compositions at earlier stages of magma evolution, whereas highly radiogenic biotite xenocrysts and the development of isotopic heterogeneity in matrix melt glass appear to coincide with the final stage of the evolution of the Fish Canyon magma body in the upper crust. Integrated petrographic and geochemical evidence is consistent with pre-eruptive thermal rejuvenation of a near-solidus mineral assemblage from ∼720 to 760°C (i.e. partial dissolution of feldspars + quartz while hornblende + titanite + biotite were crystallizing). Assimilation and blending of phenocrysts, antecrysts and xenocrysts reflects chamber-wide, low Reynolds number convection that occurred within the last ∼10 000 years before eruption. [ABSTRACT FROM AUTHOR]

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km³ of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ~0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (~39 vol%) and melt with a dacitic (~70 wt% SiO2) composition. The felsic magma mixed with ~10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting. [ABSTRACT FROM AUTHOR]

Tholeiitic basalts and trachytes of the bimodal association of Wangtian'e volcano are studied. It is shown that trachytes formed with the leading role of crystal fractionation of the parental tholeiitic magma. It has been identified from the melt inclusion study that hedenbergite and plagioclase phenocrysts of trachytes from Wangtian'e volcanic neck crystallized in the temperature range of 1080–1100 and 1050–1060°C, respectively. After heating experiments with melt inclusions in plagioclase of trachytes, in some cases relics of carbonates and CO2 were observed in the inclusions. The trachyte phenocrysts host water-bearing Fe-rich globules and CO2 inclusions with carbonate phases. The Fe-rich silicate globules are often intergrown with titanomagnetite and are covered by amorphous carbon films. A model is suggested explaining the formation of water-bearing Fe-rich globules and CO2 inclusions with carbonate relics as a result of silicate–silicate and silicate–carbonate liquid immiscibility, that was caused by the evolution of the parental basaltic melt. Upon the trachytic melt ascending to the surface, the ferrocarbonate liquid was decomposed on magnetite, carbon, and carbon dioxide. [ABSTRACT FROM AUTHOR]

Post-collisional potassic magmatic rocks are widely distributed in the northwestern Tibetan Plateau, yet their magmatic processes remain poorly understood. Here, we present a comprehensive analysis of the whole-rock major and trace elements, as well as the mineral textures and chemistry of the Dahongliutan volcanic rocks in the NW Tibetan Plateau, aiming to reveal the magmatic processes prior to eruption and speculate on the triggering mechanism. The results show that the Dahongliutan volcanic rocks are potassic trachyandesites, which undergo polybaric crystallization during magma ascension. The phenocrysts in these potassic rocks exhibit various textural and compositional zoning styles. The green cores of green-core clinopyroxenes show textural (e.g., resorption texture) and chemical (Fe-rich) disequilibrium with the host rock compositions, suggesting that they may be antecrysts and crystallized from early batches of more evolved magmas. Additionally, alkali feldspar phenocrysts also display disequilibrium characteristics (e.g., overgrowth rim and sieve texture), indicating hot mafic magma recharge and mixing in the magma plumbing system. Therefore, we conclude that the disequilibrium textural and compositional features of green-core clinopyroxene and alkali feldspar phenocrysts provide evidence of magma recharge and mixing prior to eruption. Furthermore, it is likely that the eruption of the Dahongliutan volcano was triggered by magma recharge. [ABSTRACT FROM AUTHOR]

Permo-Carboniferous rhyolitic rocks are widespread in the NE German Basin and NW Polish Basin. Hafnium (Hf) and oxygen (O) isotopes analysed in zircon from these rocks suggest diverse sources and processes involved in the formation of rhyolitic magmas. In this study, detailed core-to-rim trace element compositions were analyzed in zircon from four localities that were previously analyzed for Hf and O isotopes. The trace element analyses, in particular Hf concentrations as well as Eu/Eu*, Ce/U, Yb/Gd, and Th/U ratios, are consistent with prolonged magma evolution in three localities from the NE German Basin (Fehmarn, Slazwedel and Penkun). The fourth locality within the NW Polish Basin (Wysoka Kamieńska) is consistent with a shorter period of magma evolution. Similar stages were distinguished in zircon from the three NE German Basin localities that include: early crystallization followed by rejuvenation with more primitive magma (stage A), subsequent fractional crystallization (stage B) and finally late crystallization in a saturated system or alternatively late rejuvenation with a more primitive magma (stage C). Interestingly magmatic rims on inherited zircon grains have compositions typical for late stage B and stage C, which is consistent with their late addition to evolving rhyolitic magma, most probably during assimilation and not during source melting. The zircon from the fourth, NW Polish Basin locality shows limited compositional variability consistent with the eruption of hot magma not long after the zircon started crystallizing. Thus trace element analyses in zircon provide a record of magmatic processes complementary to that of Hf and O isotope analysis, in that, a detailed analyses of core-to-rim compositional variations are particularly useful in distinguishing respective stages of magma evolution and can pinpoint the relative timing of inherited grains being incorporated into magma. [ABSTRACT FROM AUTHOR]

At least 28 rhyolitic lava flows, domes, and tuffs erupted within Yellowstone caldera following its formation 631 ka. Understanding the timing of intracaldera eruptions is essential for characterizing natural hazards posed by Yellowstone volcano. We present 40Ar/39Ar eruption ages for the Mallard Lake Member and Central Plateau Member of the Plateau Rhyolite, which comprise products from 22 eruptions spanning ~160 to ~70 ka. These eruptions occurred along two linear vent zones within Yellowstone caldera that appear to be extensions of major extracaldera normal faults. Eruptions took place in five brief clusters with group mean ages of 160.3±1.0 ka (2σ), 149.8±4.0 ka, 111.4±0.9 ka, 104.1±0.8 ka, and 70.8±0.7 ka. All mapped products within each informal group have indistinguishable 40Ar/39Ar eruption ages implying brief eruption durations. Eruption products of each group are spatially clustered, with only one of the two linear vent zones being active during an eruption group. Using the oldest group (160 ka), we apply paleomagnetic and geochemical analyses to investigate the duration of these eruption groups and their pre-eruptive magma reservoir configuration. Paleomagnetic analyses suggest that the nine rhyolite eruptions ~160 ka spanned 400 years or less. Glass and sanidine compositions of each rhyolite are nearly identical, which is consistent with pre-eruptive storage of a large (~130 km3), interconnected melt body within Yellowstone volcano's crystal-rich magma reservoir. These characteristics differ from intracaldera eruptions that vented near the caldera's inner ring fracture system, which tend to be compositionally disparate with less temporal clustering. These insights inform hazard mitigation scenarios. [ABSTRACT FROM AUTHOR]

The world-class Jinchuan magmatic Ni-Cu-PGE (platinum-group element) sulfide deposit comprises four large orebodies (No. 1, 2, 3 and 24). Underground fan drilling discovered significant lateral extension of No. 2 orebody, referred as No. 2a in this study. From base to top, the No. 2a orebody comprises disseminated sulfide in olivine pyroxenites, net-textured ore in lherzolite, and massive ore. Despite different host rocks, the contents of Ni, Cu and PGE of the same types of sulfide ores from No. 2 and No. 2a orebodies are comparable. Mass balance calculation indicates that the parental magmas contained 0.01–0.05 ppb Ir and 0.2-0.8 ppb Pd, which are about one order of magnitude less than PGE abundances of undepleted continental flood basalts. PGE tenors (recalculated to 100% sulfides) of the net-textured ores from both orebodies increase upward. We attribute this to variations in magma:sulfide ratios of successive batches of recharged magmas and/or reaction of sulfides with less evolved magma. We propose that the No. 2a and No. 2 orebodies were located in the upstream and downstream parts of an originally sub-horizontal magma conduit, respectively. Sulfide liquids accumulated in the wider parts of the magma conduit to form No. 2a and No. 2 orebodies progressively. Variable locations within the intrusion and sharp contacts with other types of sulfide ore indicate that the massive ores formed by injection of pooled sulfide melt. Significant Pt-depletion of the massive ores and Pt enrichment in the adjacent net-textured ores suggest migration of residual fractionated sulfide liquids. [ABSTRACT FROM AUTHOR]

Plagioclase phenocrysts in pre-historic andesites provide insight to the dynamics of magma formation and eruption at Taranaki volcano, New Zealand. The phenocryst population has a diversity of relic cores and a total in situ 87Sr/86Sr range of 0.70440–0.70486. Within-sample 87Sr/86Sr variations of 0.00018 to 0.00043 indicate that many phenocrysts are antecrysts and/or xenocrysts, derived from multiple crystal mush bodies. The Sr-isotopic differences in the phenocrysts of consecutive eruptions indicate that different magmas were tapped or formed on a centennial timescale. Most phenocrysts have multiple resorption/calcic regrowth zone(s) with elevated FeO* but invariant MgO zonation profiles. They likely record mafic melt inputs, and subsequent storage at elevated temperature caused re-equilibration of the Mg gradient. However, distinct rim types record different final pre/syn-eruptive magmatic conditions. Those in magmas erupted at 1030–1157 CE, 1290–1399 CE and 1780–1800 CE are characterised by resorption and calcic regrowth with sharp MgO and FeO* gradients. They record the entry of mafic melt into the system a few days or less before eruption based on Mg diffusion chronometry. In contrast, most phenocrysts erupted at 1755 CE, 1655 CE, and a few pre-1 ka events, have texturally uniform rims, compositionally consistent with closed-system crystallisation. This suggests alternating external and internal eruption triggers. Alternatively, the rate of magma reactivation via intrusion may dictate whether there was sufficient time for a mineralogical response to be recorded in part or all of the system. With respect to anticipating future eruptions, the plagioclase phenocrysts suggest multi-stage magma priming but rapid onset of the final trigger. [ABSTRACT FROM AUTHOR]

Arc magmatism plays a key-role in the growth and differentiation of continental crust. In particular, the prolonged magmatic flux events control the genesis of deep crustal hot zones where magma accumulation and fractionation favour the continental arc crust evolution and geochemical stratification. In this view long-lived magmatic systems located in arc domains, with their erupted products spanning from basalts to rhyolites are formidable archive for the understanding the evolution of transcrustal magmatic plumbing systems and the construction of a stratified continental crust. This study focuses on the long-lived (ca. 9 Ma) Miocene-Holocene Cordillera de San Buenaventura volcanic system in the Southern Puna Plateau (NW Argentina), a unique natural laboratory where exploring the building up of MASH-zones (or deep crustal 'hot zones') and related sustained volcanism. Synthesis of new and published data, together with new thermobarometry and fractional crystallization modelling, indicates a cyclic scenario with mantle melts undergoing fractional crystallization dominated incipient and waning stages alternate to major mafic magma recharge events during the building up phase of the MASH-zone. This magmatic scenario is also discussed in the light of the coeval geodynamic framework dominated by the subduction of the Nazca plate and the eastward migration of the frontal arc magmatism. [ABSTRACT FROM AUTHOR]

Liquid immiscibility has become the preferred mode of genesis for the carbonatite rocks, which commonly, but not exclusively, accompany silicate rocks in alkaline-rock complexes. This concept has been universally based on the presumption that nephelinitic and phonolitic magmas can evolve to a stage where two conjugate immiscible liquids separate. It is assumed that these two liquids separate quickly, or even instantaneously, into discrete bodies of magma capable of being intruded or extruded with subsequent independent crystallization. Supporting evidence generally given is: alleged consanguinity as discrete occurrence of the two rock types; similarity of radiogenic isotope ratios; trace element contents similar to those predicted from experimentally derived partition coefficients. We do not accept that a general case for liquid immiscibility has been demonstrated; although we do accept that silicate and carbonate liquids are inherently immiscible, we maintain that they are not conjugate in a petrogenetic context. We have reviewed and critically examined the experimental data purporting to establish liquid immiscibility and find that when applied to natural rocks, they are based on inappropriate experimental designs, which are not relevant to the genesis of calcite or dolomite carbonatites, although they might have some relevance to Oldoinyo Lengai nyerereite–gregoryite lavas. The design of these experiments guarantees immiscibility and ensures that the carbonate liquids formed will be calcitic or sodium-rich. We dispute the validity of comparing the trace element contents of natural rocks, which in many instances do not represent liquid compositions, to experimentally determine partition coefficients. We consider that experimental design inadequacies, principally assuming but not proving, that the liquids involved are conjugate, indicate that these coefficients are merely an expression of the preference of certain elements for particular liquids, regardless of how the liquids formed. Proof of consanguinity in alkaline complexes requires more accurate age determinations on the relevant rock types than has generally been the case, and in most complexes, consanguinity can be discounted. We dispute the contention that melt inclusions represent parental melts, although they might elucidate the character of magmas undergoing fractional crystallization from magmatic to carbothermal stages. Radiogenic isotope data are shown to be too widely variable to support a case for liquid immiscibility. We address the contention that calcite cannot crystallize from a dolomitic liquid formed by direct mantle melting, and must therefore have crystallized from a calcite carbonate liquid generated by liquid immiscibility, and demonstrate that it is an unsupported hypothesis as calcite can readily crystallize from dolomitic liquids. We observe that, because immiscible dolomite liquids have never been produced experimentally, the liquid immiscibility proposition could at best be applied only to calcite carbonatites, thus leaving unexplained the large number of dolomite carbonatites and those of either type, which are not accompanied by alkaline silicate rocks. The assumed bimodality of alkaline-rock carbonatite complexes is considered to be fallacious and no actual geological or petrographic evidence for immiscibility processes is evident in these complexes. Several examples of alkaline rock carbonatite complexes for which immiscibility has been proposed are evaluated critically and shown to fail in attempts to establish them as exemplifying immiscibility. We conclude that no actual geological or experimental data exist to establish liquid immiscibility being involved in the genesis of calcite or dolomite carbonatite-forming magmas. [ABSTRACT FROM AUTHOR]

Mount Elbrus, Europe's tallest and largely glaciated volcano, is made of silicic lavas and is known for Holocene eruptions, but the size and state of its magma chamber remain poorly constrained. We report high spatial resolution U–Th–Pb zircon ages, co-registered with oxygen and hafnium isotopic values, span ~ 0.6 Ma in each lava, documenting magmatic initiation that forms the current edifice. The best-fit thermochemical modeling constrains magmatic fluxes at 1.2 km3/1000 year by hot (900 °C), initially zircon-undersaturated dacite into a vertically extensive magma body since ~ 0.6 Ma, whereas a volcanic episode with eruptible magma only extends over the past 0.2 Ma, matching the age of oldest lavas. Simulations explain the total magma volume of ~ 180 km3, temporally oscillating δ18O and εHf values, and a wide range of zircon age distributions in each sample. These data provide insights into the current state (~ 200 km3 of melt in a vertically extensive system) and the potential for future activity of Elbrus calling for much-needed seismic imaging. Similar zircon records worldwide require continuous intrusive activity by magmatic accretion of silicic magmas generated at depths, and that zircon ages do not reflect eruption ages but predate them by ~ 103 to 105 years reflecting protracted dissolution–crystallization histories. [ABSTRACT FROM AUTHOR]

Mafic magmatic enclaves (MMEs) hosted in granitic plutons are ideal to investigate the role of mushes on magma mixing processes in crustal magma chambers. However, the petrographic evidence for mixing of magmas through infiltration and percolation in coexisting mushes and magmas is desired. Here, we describe complex zoning patterns of plagioclase in the MMEs hosted in the monzogranite of the Early Cretaceous Sanguliu pluton in East China to reveal a mush-facilitated magma mixing process. The MMEs appear as round to oval nodules about 10 to 20 cm in size and show diverse disequilibrium textures. Plagioclase in the MMEs can be identified as three populations (Plag1, Plag2, and Plag3) with distinct zoning patterns, anorthite contents (X An) and initial Sr isotopic ratios (87Sr/86Sri). Plag1 is antecryst displaying normal zoning with An42–67 in the core and An20–36 in the mantle. The core of Plag1 shows coarse sieve texture with high-frequency oscillation in the margin, and the mantle displays resorption surface and patchy zoning. Plag2 is also antecryst with An23–66 in the core and An21–35 in the mantle. However, its core can be further recognized as Core I inside and Core II outside with distinctly different An23–43 and An44–66, respectively, showing reverse zoning. In addition, Core I contains aligned biotite inclusions and Core II shows sieve texture, resorption surface and patchy zoning. Amphibole inclusions are sporadically enclosed within Core I of Plag2 (Amp1) and mantles of Plag1 and Plag2 (Amp2), but rarely observed in Core II of Plag2. Plag3 is anhedral grain in the matrix and shows core-rim texture with An20–37 in the core. The three plagioclase populations all exhibit angular rims with resembling An9–22. Plag1 core and Plag2 Core II have (87Sr/86Sr)i (0.70920 to 0.71092) similar to the bulk (87Sr/86Sr)i of the mafic dykes intruding the Sanguliu pluton, and likely crystallized from basaltic andesitic magmas. In contrast, the rims of Plag1, Plag2, and Plag3 overall have (87Sr/86Sr)i (0.71391 to 0.71583) nearly identical to the (87Sr/86Sr)i of host monzogranite and the plagioclase in the monzogranite, likely crystallized from granitic magmas. The mantles of Plag1 and Plag2 and the core of Plag3 have (87Sr/86Sr)i (0.71141 to 0.71390) overlapping the (87Sr/86Sr)i of the MMEs, and may have crystallized from mixed melts. Calculation results based on amphibole thermobarometers show that Amp1 crystallized at ~775 °C and ~ 16 km depth, whereas Amp2 and the amphibole in the matrix of the MMEs and monzogranite crystallized at 730 to 744 °C and 8 to 9 km depth. We thus propose that the chemical and textural complexity of the three plagioclase populations in the MMEs can be attributed to that the MMEs may have come from a mushy hybrid layer that was developed through a molten granitic body being recharged by upwelling basaltic andesitic magma. Core I of Plag2 may have nucleated and grown from andesitic magma that was evolved from the basaltic andesitic magma from which the core of Plag1 and Core II of Plag2 crystallized. The two types of antecrystic plagioclase then may have experienced resorption and disequilibrium growth in the hybrid layer, and finally rimmed with ambient, evolved interstitial melt within mushy MMEs. This study shows that complex zoning patterns and compositions of plagioclase populations in the MMEs hosted in granitic plutons have important bearings on mush-facilitated magma mixing processes. [ABSTRACT FROM AUTHOR]

Mount Elbrus, Europe's tallest and largely glaciated volcano, is made of silicic lavas and is known for Holocene eruptions, but the size and state of its magma chamber remain poorly constrained. We report high spatial resolution U–Th–Pb zircon ages, co-registered with oxygen and hafnium isotopic values, span ~ 0.6 Ma in each lava, documenting magmatic initiation that forms the current edifice. The best-fit thermochemical modeling constrains magmatic fluxes at 1.2 km3/1000 year by hot (900 °C), initially zircon-undersaturated dacite into a vertically extensive magma body since ~ 0.6 Ma, whereas a volcanic episode with eruptible magma only extends over the past 0.2 Ma, matching the age of oldest lavas. Simulations explain the total magma volume of ~ 180 km3, temporally oscillating δ18O and εHf values, and a wide range of zircon age distributions in each sample. These data provide insights into the current state (~ 200 km3 of melt in a vertically extensive system) and the potential for future activity of Elbrus calling for much-needed seismic imaging. Similar zircon records worldwide require continuous intrusive activity by magmatic accretion of silicic magmas generated at depths, and that zircon ages do not reflect eruption ages but predate them by ~ 103 to 105 years reflecting protracted dissolution–crystallization histories. [ABSTRACT FROM AUTHOR]

We integrate textural and in situ compositional information from plagioclase and clinopyroxene (Cpx) phenocrysts together with groundmass compositions in early Cretaceous andesite dykes within the Sulu belt of China to propose a new petrogenetic model for andesite. Plagioclase phenocrysts are mostly andesine; they are depleted in high field strength elements (HFSE). However, Cpx phenocrysts are either reversely zoned (type I) or homogeneous (type II), with the zoned Cpx divided into subtypes IA and IB. All Cpx has high Mg#, low Na2O and generally low Al2O3, with depletions in HFSE and variably high 87Sr/86Sr ratios, suggesting crystallization above the Moho from magmas derived from enriched lithospheric mantle. The cores of type IA/IB and type II Cpx have normal major‐ and trace‐element compositional variations and similar 87Sr/86Sr ratios to each other and to plagioclase, consistent with fractional crystallization from a common magma (magma 1). The rims of type IA and IB Cpx also have normal major‐ and trace‐element compositional variations, but these are not as evolved as the cores, and the rims have lower 87Sr/86Sr ratios, demonstrating crystallization from an isotopically distinct magma (magma 2). Based on modeled major and rare earth element compositions of magmas inferred to have been in equilibrium with different Cpx (±plagioclase) domains, the measured groundmass compositions can be reproduced by variable mixing between the two magmas. Our study demonstrates for the first time that andesite magma can be made through fractionation and shallow hybridization of magmas derived from variably enriched lithospheric mantle. Plain Language Summary: Andesite is the characteristic volcanic rock that is formed where Earth's tectonic plates are thrust one under another, as occurs along the Andes, the Cordillera of Central and North America, and the western margin of the Pacific Ocean, but how the magmas form remains controversial. In the mantle model, an enriched source is thought to form by reaction between mantle peridotite and metamorphic fluids derived from seafloor sediments and/or altered oceanic lithosphere during under‐thrusting of an ocean plate beneath a continental margin. By contrast, the shallow model proposes that andesite magmas are formed by differentiation of basaltic magmas or mixing of magmas from mantle and crustal sources in the lithosphere of an overriding continental plate. In either case, the composition of the magma may evolve due to crystal fractionation or be modified by crustal assimilation. Here, we use petrographic observations and in situ major‐ and trace‐element, and Sr isotope compositions of plagioclase and clinopyroxene phenocrysts and groundmass in early Cretaceous andesite dykes from the Sulu belt of China, to show for the first time that andesite can be made through fractionation and shallow hybridization of two different magmas derived from variably enriched lithospheric mantle of the underlying continental plate. Key Points: Using compositions of plagioclase/clinopyroxene phenocrysts and the groundmass in andesite we develop a new model of andesite petrogenesisThis model requires polybaric crystallization and hybridization between 2 enriched lithospheric mantle‐derived magmas at crustal depthWe demonstrate that andesite of similar composition to continental crust can be made through mixing magmas derived from the enriched subcontinental lithospheric mantle [ABSTRACT FROM AUTHOR]

Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes, such as magma differentiation, magma recharge and mixing, and crustal contamination. The plagioclase phenocrysts in the Daqiao and Qiaojia plagioclase-phyric basalts from the Emeishan Large Igneous Province (LIP) show complex textural and compositional zoning patterns, e.g., normal, reverse, oscillatory, and patchy zoning patterns. Most plagioclase phenocrysts exhibit a core–rim normal zoning pattern (Pl-A) with euhedral high-An cores (An = 76–78%, in mole fraction) and low-An rims (An = 68–72%), indicative of the crystal regrowth processes caused by recharge of relatively evolved magmas after the formation of high-An cores. Some phenocrysts have a core–rim reverse zoning pattern (Pl-B) with irregular ovaloid cores, characterized by extremely low An (60–61 mol%) and Ba (84–88 ppm) contents and extremely high 87Sr/86Sr ratios (0.7120–0.7130). The rims of the Pl-B have relatively high An (69–72 %), Ba (~160 ppm) contents, and low 87Sr/86Sri (~0.7056). These Pl-B plagioclase phenocrysts preserve the information about the interaction between the crustal xenocrysts and the transporting magmas. Some plagioclase phenocrysts show a core–mantle–rim oscillatory zoning pattern (Pl-C) with multiple oscillations of An (70–80 %), Ba (88–147ppm) from core to rim, revealing replenishment and mixing of multiple batches of basaltic melts with diverse compositions. 87Sr/86Sr ratios of the Pl-C do not vary significantly (0.7050–0.7054). A small portion of phenocrysts has patchy patterns in the cores (Pl-D), where the low-An patches (72–75 %) in form of elliptical or irregular elongated shapes were enclosed by the high-An domains (80–87 %). These features can be attributed to crystal dissolution and regrowth processes during the reaction between early-formed low-Cumulates and recharged hot primitive melts. The cores, mantles, and rims of different types of plagioclase phenocrysts (except the core of Pl-B) commonly display nearly constant Sr isotopic compositions, implying insignificant wall-rock assimilation at shallow-level magma reservoir(s) during the growth of these plagioclase phenocrysts. In conclusion, the massive crystallization of plagioclase in the late stage was an important controlling factor for the formation of iron-rich basalts in the Emeishan LIP. [ABSTRACT FROM AUTHOR]

Centimeter-scale magmatic enclaves are abundant in peralkaline felsic volcanic rocks in the Sanshui Basin. Their lithology is mainly syenite and syenitic porphyry, and they mainly comprise alkali feldspar and amphibole, which is similar to the mineral assemblage of the host trachyte and comendite. The SiO2 content in the syenitic enclaves is ~63 wt%, which is similar to that of the host trachyte but lower than that of the comendite. Thermobarometric calculations showed that the syenitic enclaves crystallized at similar temperature and pressure conditions as their host trachyte. The results of mass-balance modeling and MCS modeling indicate that the syenitic enclaves likely experienced an approximately 74% fractional crystallization from the basaltic parental magma. Combined with the similar mineral assemblages and geochemical characteristics of the host trachyte, we think that the enclaves resulted from the in situ crystallization of trachytic magma in the shallow crust and that they had a cogenetic origin with their host volcanic rocks, which means that they were likely to derived from the identical magma chamber which was formed from different batches of magma mixing/mingling. The recharge and mixing of basaltic magma triggered the eruption of trachytic magma eruption. The syenitic crust may have been disaggregated by the ascending trachytic magma and brought to the surface as syenitic enclaves. The syenitic enclaves in volcanic rocks provide unique information on the magmatism of the shallow crust as evidence of magma mixing/mingling. [ABSTRACT FROM AUTHOR]

We use comprehensive geochemical and petrological records from whole-rock samples, crystals, matrix glasses and melt inclusions to derive an integrated picture of the generation, accumulation and evacuation of 530 km of crystal-poor rhyolite in the 25.4 ka Oruanui supereruption (New Zealand). New data from plagioclase, orthopyroxene, amphibole, quartz, Fe-Ti oxides, matrix glasses, and plagioclase- and quartz-hosted melt inclusions, in samples spanning different phases of the eruption, are integrated with existing data to build a history of the magma system prior to and during eruption. A thermally and compositionally zoned, parental crystal-rich (mush) body was developed during two periods of intensive crystallisation, 70 and 10-15 kyr before the eruption. The mush top was quartz-bearing and as shallow as ~3.5 km deep, and the roots quartz-free and extending to >10 km depth. Less than 600 year prior to the eruption, extraction of large volumes of ~840 °C low-silica rhyolite melt with some crystal cargo (between 1 and 10%), began from this mush to form a melt-dominant (eruptible) body that eventually extended from 3.5 to 6 km depth. Crystals from all levels of the mush were entrained into the eruptible magma, as seen in mineral zonation and amphibole model pressures. Rapid translation of crystals from the mush to the eruptible magma is reflected in textural and compositional diversity in crystal cores and melt inclusion compositions, versus uniformity in the outermost rims. Prior to eruption the assembled eruptible magma body was not thermally or compositionally zoned and at temperatures of ~790 °C, reflecting rapid cooling from the ~840 °C low-silica rhyolite feedstock magma. A subordinate but significant volume (3-5 km) of contrasting tholeiitic and calc-alkaline mafic material was co-erupted with the dominant rhyolite. These mafic clasts host crystals with compositions which demonstrate that there was some limited pre-eruptive physical interaction of mafic magmas with the mush and melt-dominant body. However, the mafic magmas do not appear to have triggered the eruption or controlled magmatic temperatures in the erupted rhyolite. Integration of textural and compositional data from all available crystal types, across all dominant and subordinate magmatic components, allow the history of the Oruanui magma body to be reconstructed over a wide range of temporal scales using multiple techniques. This history spans the tens of millennia required to grow the parental magma system (U-Th disequilibrium dating in zircon), through the centuries and decades required to assemble the eruptible magma body (textural and diffusion modelling in orthopyroxene), to the months, days, hours and minutes over which individual phases of the eruption occurred, identified through field observations tied to diffusion modelling in magnetite, olivine, quartz and feldspar. Tectonic processes, rather than any inherent characteristics of the magmatic system, were a principal factor acting to drive the rapid accumulation of magma and control its release episodically during the eruption. This work highlights the richness of information that can be gained by integrating multiple lines of petrologic evidence into a holistic timeline of field-verifiable processes. [ABSTRACT FROM AUTHOR]

Fractional crystallization of Fe–Ti oxides can induce detectable Mg isotopic changes during late-stage basalt differentiation. Because chromite and olivine are early crystallizing phases during basaltic melt differentiation, the effect of chromite crystallization on the fractionation of Mg isotopes during early-stage basalt differentiation is still poorly understood. Here, we examine the possibility of chromite induced Mg isotope fractionation with a Mg isotopic study of chromite–olivine pairs in dunites and chromitites collected from major types of basaltic intrusive rock suites formed by fractional crystallization in different tectonic settings. The chromite δ26Mg (= [(26Mg/24Mg)sample / (26Mg/24Mg)DSM3–1] × 1000) values range from −0.19‰ to 0.30‰ in the Luobusa ophiolite, −0.09‰ to 0.78‰ in the Kızıldağ ophiolite, −0.04 to 0.42‰ in the Gaositai Alaskan-type complex, similar to those previously reported from the Stillwater layered intrusion (−0.05 to 0.84‰; Bai et al., 2021). They are significantly higher than those of coexisting olivine (δ26Mg = −0.48 to −0.10‰). The Δ26MgChr-Ol (= δ26MgChr − δ26MgOl) values in the rock suites investigated here fall largely between equilibrium fractionation lines of spinel–olivine pairs and magnesioferrite–olivine pairs, indicating equilibrium Mg isotopic fractionation. Furthermore, the Δ26MgChr-Ol values increase with decreasing Cr content of chromite in the dunites and chromitites, showing that high 26Mg has a greater affinity to Al-rich chromite than Cr-rich chromite. Fractional crystallization of such isotopically high chromite is expected to progressively lower the Mg isotope values of the in the remaining magma. Furthermore, continental basaltic magmas typically experience early crystallization of olivine and Al-rich chromite. Their δ26Mg values correlate positively with MgO (FeO, Cr,) and CaO/Al2O3 ratios and negatively with total alkali contents (Na2O + K2O). This indicates that detectable Mg isotopic fractionation occurred in intra-continental basalt magmas, probably by fractional crystallization of olivine and chromite. The observed low-δ26Mg intra-continental basalts can be accurately modeled by olivine + chromite fractionation with fractionation factors (Δ26MgChr-Melt) of 0.20‰, 0.60‰, and 1.18‰ as observed in the chromitites investigated during this study. Therefore, the early-stage basaltic melt differentiation involving separation of olivine and chromite may induce resolvable Mg isotopic fractionation, and the δ26Mg values of continental basalts should be used with caution in petrogenetic studies. [ABSTRACT FROM AUTHOR]

The origin of the anorthosite-rapakivi granite-bearing rock complexes was a topic of research for geologists for more than a century. Magmatic systems that produce these complexes were widely developed in the Precambrian and have no direct analogues in modern times. The main goal of this study was to characterize the conditions under which primitive granitoids formed and to shed light on the history of ore matter transport during these stages. The object of the research was granite porphyry dikes—a subeffusive analogue of rapakivi granite from the Salmi batholith in the Fennoscandian Shield. Characteristic solid-phase inclusions of a mineral paragenesis of mafic rocks, as well as inclusions of sulfide melts, were found in zircon from this type of rock. These sulfide inclusions were homogenized by heating, and subsequently, the trace element composition was determined by the LA-ICP-MS method. The geochemistry of zircons was studied by the SIMS method, and the temperature of their formation was determined using a Ti-geothermometer. The temperature of the studied zircon grain formation varied from 925 to 765 °C. The values of ƒO2 for the early stages of the rock formation were in the region below the boundary of the FMQ buffer. The presence of two sulfide melts at the time at which the sulfide inclusions were captured by the zircon, pyrrhotite-chalcopyrite, and sphalerite-chalcopyrite compositions was established. The capture of sulfide inclusions from the pyrrhotite-chalcopyrite composition occurred above 841 °C, while in sphalerite-chalcopyrite, they occurred at a lower temperature of 765 °C. The connection between the formation of granite porphyry with mafic melts that form rocks of the gabbro and anorthosite types was established. [ABSTRACT FROM AUTHOR]

Giant volcanic eruptions have the potential to overturn civilizations. Yet, the driving mechanism and timescale over which batholithic magma reservoirs transition from non‐eruptible crystal mush to mobile melt‐dominated stages and our capacity to detect a pending super‐eruption remain obscure. Here we show, using Sr isotope zonation in plagioclase crystals from three Andean large‐magnitude eruptions (Atana, Toconao, and Tara ignimbrites), that eruptible magma forms by amalgamation of isotopically diverse crystal populations and silicic melt without large‐scale reheating. In each case, crystals record large isotopic diversity in crystal cores, converging toward a common value in crystal rims that coincides with the composition of the rhyolitic carrier melt. Using diffusion chronometry, we show that the assembled magma resided pre‐eruptively in the crust for timescales of no more than decades to centuries for Atana and Tara, but up to several millennia for Toconao. These timescales and isotopic observations are consistent with the accumulation and destabilization of melt‐rich layers in crystal mush. While the prospect of capturing such melt lenses with most geophysical monitoring techniques is pessimistic, gravity modeling indicates that such structures are potentially resolvable. Our findings provoke a new assessment of the origin and hazards associated with large magnitude explosive eruptions. Plain Language Summary: Super‐eruptions are the largest manifestation of explosive volcanism on Earth. If such an event would occur today, it could deeply impact human civilization on a global scale. Yet, signals that may emerge prior to such eruptions are difficult to constrain. To gain greater understanding of the mechanisms and timescales over which giant, eruptible, reservoirs form, and to evaluate our capacity of capturing the evolution toward large eruptions, we studied plagioclase crystals, a mineral that is an excellent recorder of magmatic processes. For three large eruptions in the Central Andes, we analyzed strontium isotopes in traverses from crystal cores to rims to provide a temporal record of melt composition. Our results show that, in each case, the eruptible magma amalgamated from distinct pre‐existing magma pockets that merged prior to eruption. This merged state of the magma did not exist for longer than a few hundred to thousands of years based on the preservation of the crystal strontium‐isotope traverses at high temperatures. These results are consistent with the destabilization of melt‐rich layers on timescales comparable to human lifespans. Although most geophysical imaging methods are unlikely to capture this process, our calculations indicate that gravity monitoring of super‐volcanoes can potentially resolve such structures. Key Points: Sr‐isotopes in plagioclase record the emergence of eruptible magma through amalgamation of pre‐existing mush in the Central AndesThe amalgamated magma did not reside at depth for longer than centuries to millennia based on diffusion chronometryForward modeling indicates that high‐precision gravity monitoring has large potential to resolve eruptible magma amalgamation [ABSTRACT FROM AUTHOR]

The Abu Ghalaga pluton in the southern eastern desert of Egypt (SED) consists of undeformed norite and hornblende gabbro intruding island arc metavolcanics, ophiolitic rocks, and syn-tectonic older granitic rocks. The Fe-Ti ore (mainly ilmenite) occurs in the form of lenses at the base of the norite. Based on microtextures, mineralogy, and chemical composition, three types of ilmenites are recognized: massive high-Mg type, disseminated high-Mg type, and disseminated low-Mg ilmenite. The three ilmenite types are chemically distinguished by their MgO, MnO, TiO2, Fe2O3, FeO, and V2O3 contents. The variations in the concentration of these oxides are interpreted as a function of variations in T, fO2, and the composition of the parental magma. Reconstruction of the orthomagmatic conditions shows that T and fO2 of the magma from which the norite and its Fe-Ti deposit formed decrease during the course of magmatic differentiation, whereas the hornblende gabbro and its Fe-Ti mineralization have formed from a more evolved magma under relatively lower T and fO2. The REE patterns and variation diagrams support a model of crystal fractionation as the primary means of formation of the Fe-Ti deposits. Chemical features also suggest that the parent magma of both types of gabbroic rocks was generated in the mantle, but in two different pulses. Subduction zone, MORB, and WPB chemical signatures for these gabbroic rocks suggest a complex tectonic history consistent with compression followed by within plate extension possibly contributing to the development of a back-arc/intra-arc basin. [ABSTRACT FROM AUTHOR]

In central Victoria, inherited zircon in Devonian igneous rocks and detrital zircon in metasedimentary country rocks and an amphibolite-facies xenolith show that Mesoproterozoic parts of the underlying Selwyn Block cannot be the source for all the silicic magmas. Zircon inheritance in S-type samples reveals significant thermal events at 525–425 Ma and 1200–1100 Ma. Both S- and I-type samples have prominent zircon age peaks at 420–410 Ma, which record high-grade metamorphism of the deep crust during the terminal phases of the Benambran and Bindian orogenies. All I-type rocks have 650–500 Ma peaks, suggesting derivation from an arc-related metavolcanic source in the upper Selwyn Block. Protoliths of the greenschist-facies Ordovician metasediments and the amphibolite-facies Cambrian metasedimentary xenolith were deposited in distal backarc settings. Most inherited zircon cores are metamorphic, and the strongest zircon inheritance occurs in hornblende-bearing I-type rocks, highlighting their largely crustal origin. Zircon populations at ca 1400 Ma, thought to signal sediment derivation from East Antarctica and Rodinia-Nuna, are mostly absent in I-type samples and some S-types. The ca 1400 Ma signal probably applies to the upper, metasedimentary Selwyn Block, so Devonian S-type magmas were sourced mainly in the deeper sections. Zircon inheritance in the Devonian igneous rocks was not influenced by the exposed metasedimentary country rocks. Two samples from one of the smaller plutons have contrasting patterns of zircon inheritance, suggesting relatively small-scale source heterogeneity. Many rounded and corroded cores in zircon crystals yield the same ages as the crystallisation dates for the rocks, and thus are antecrysts. Higher whole-rock Zr contents generally correlate with higher proportions of inherited zircon, and differentiation does not affect this relationship. The degree of partial melting of a magma source and the efficiency of crystal entrainment are critical in governing zircon inheritance. Mesoproterozoic sections of the Selwyn Block cannot be the sources for all the Devonian silicic magmas in central Victoria. I-type rocks have 650–500 Ma zircon age peaks, suggesting derivation from arc-related metavolcanic rocks in the upper Selwyn Block. Hornblende-bearing I-type rocks have the strongest zircon inheritance patterns, indicating the largely crustal origins of I-type magmas. Exposed metasedimentary country rocks were not involved in magma genesis. [ABSTRACT FROM AUTHOR]

Volcanism is the surface expression of extensive magmatic systems, with their intrusive counterpart representing ~80% of the total magma budget. Our knowledge of igneous processes therefore largely relies on our understanding of deep plutonic processes. In continental or oceanic environments, most of the intrusive igneous rocks bear geochemical cumulate signatures (e.g. depletion in incompatible elements and enrichment in compatible ones) that are commonly explained by mineral-melt segregation during differentiation. Deformation-assisted compaction aided by melt buoyancy is usually referred to as the main process involved in melt extraction. However, buoyancy alone is not sufficient, and a number of cumulative rocks are lacking any compaction evidence, opening the potential for the involvement of other processes. In addition, our view of magmatic systems has shifted in the last decades from large melt-rich bodies to crystal-rich magma reservoirs. This paradigm shift challenges some of the long-established first-order igneous concepts like the idea that melt differentiation at depth is mainly governed by (fractional) crystallization; alternatively, the presence of mush potentially favors additional processes such as melt-mush reactions. We propose a novel igneous process for the formation of igneous cumulates, consistent with the mushy nature of oceanic igneous reservoirs, their continuous/cyclic replenishment by primitive melts, and the widespread occurrence of reactive porous flow (RPF) during magma differentiation identified in a growing number of magmatic systems. The melt flush process relies on melt-mush reactions between the primitive recharge melt(s) and crystal mush. Replacement of the more evolved interstitial melt by the primitive recharge melt leading to reactions (dissolution+crystallization) and concomitant extraction of the more evolved melt from the cumulate by buoyancy participate in the acquisition of the final cumulate signature. This process relying on oceanic igneous systems considers for the first time melt inputs and not only melt extraction and matches the petrographic (e.g. mineral dissolution evidence) and geochemical constraints (trace element signatures) brought by natural oceanic samples. We tested various melt-mush reactions likely involved in the early stages of the melt flush process during RPF to investigate their thermodynamic feasibility with the Magma Chamber Simulator. First-order results show that one-step equilibration of primitive melts with primitive to moderately differentiated mush crystals triggers mineral assimilation. Together with the constraints established from the natural rock record, it strengthens the idea that RPF is a potential key process for magma differentiation in magma reservoirs at different evolution stages. The proposed melt flush process eventually adds to other processes involved in cumulate formation like magma compaction or crystal settling and is likely to apply to any other magmatic system from various settings sharing similar reservoir characteristics. [ABSTRACT FROM AUTHOR]

Understanding the petrogenesis of alkaline syenites is important for constraining the mechanisms of rare earth element (REE) and rare-metal mineralization. Here we report a detailed petrological and geochemical study of early Permian syenitic rocks from the southwestern Tarim large igneous province (TLIP) in the Wajilitag area (China). We use these data to investigate the complex magmatic processes responsible for the enrichment of incompatible elements (e.g. REEs and rare metals) in these rocks. The Wajilitag syenitic rocks comprise early hornblende syenite (281 Ma) and later nepheline syenite (278 Ma), both of which are spatially and temporally associated with mafic intrusions (i.e. gabbro and diabase). These syenitic rocks show continuous variations in major elements with the mafic rocks. They also have similar Sr–Nd–Hf isotopic compositions with the nearby mafic rocks, denoting that the Wajilitag syenitic rocks should be derived from the melts represented by these mafic rocks. However, these syenitic rocks, particularly the nepheline syenites, are typically characterized by extreme enrichment of incompatible elements, which is hard to be accounted by simple fractional crystallization of mafic magmas. Some clinopyroxene phenocrysts in the Wajilitag nepheline syenites show oscillatory zoning with strikingly increasing and then gradually decreasing MgO contents, which recorded replenishment of mafic magma at the interval. Low-MgO clinopyroxene mantles and/or rims have much higher incompatible element contents (e.g. Nb, Ce, and Zr) and ratios (e.g. Ce/Y) than their cores, which could reflect melt injection controlled by reactive porous flow in a mush reservoir of a crustal magma chamber. Such processes may also cause the high Ce and/or Nb contents of the mantles and/or rims of zoned titanite and apatite phenocrysts. The injection of reactive porous flow melts is the key process that produces the extreme enrichment of incompatible elements in the alkaline syenites from the western TLIP, as well as other plume-related alkaline syenites that host world-class REE and rare-metal deposits. [ABSTRACT FROM AUTHOR]

Magma produced by melting of continental crust and mantle at the Archean-Proterozoic boundary are compositionally variable and chemical compositions provide evidence for the mixing of two sources. Understanding the composition of hybrid magma is essential for determining the comparative influence of crust and mantle sources during orogenesis. The hybrid granites are less documented in Indian cratons, especially less in Dharwar Craton. Here we present petrographic and whole-rock geochemical data of Madgulapalli granitic rocks situated in the NE part of the Eastern Dharwar Craton (EDC), to elucidate their petrogenesis and role in crust formation. The Madugulapalli granites (MPG) are composed chiefly of plagioclase, quartz, and alkali feldspar with associated biotite showing alteration and inter-granular textures. Geochemically, they are metaluminous to peraluminous in nature with calc-alkaline hybrid granite. The hybrid granites exhibit both negative and positive europium anomalies; the lower Rb/Sr, Rb, Sr, and higher Sr/Y, (Dy/Yb)N ratios suggest that the interaction of older rocks with residual garnet source melted at high pressures. We hypothesize that hybrid granites are formed by interaction (e.g., metasomatism, mingling, or mixing) between parental magmas and pre-existing rocks with the influence of sanukitoid melts (heat source) in a subduction environment. The genesis of the hybrid granites demonstrates the mixing coupled with differentiation in the petrogeny's residue system in a syn-collision setting followed by continental crust stability in EDC during the Neoarchean period. [ABSTRACT FROM AUTHOR]

Abstract—The effect of sulfide sulfur on the crystallization of ultramafic–mafic–intermediate magmatic systems is considered in order to assess its scale and direction during the crystallization of rock-forming phases. The study carried out a theoretical analysis of the effect of sulfur on the activity of the components of the silicate melt according to the Korzhinskii's principle of acid–base interaction. To test the theoretical constructions, published experimental data analyzed to select data allowing a direct comparison of sulfur-saturated and sulfur-free runs. Despite the very limited number of such experiments, they all fit well with theoretically predicted trends. It has been established that the addition of sulfur to the system moderately reduces the liquidus temperature of rock-forming minerals within 20–30°C, and significantly affects their composition. In particular, chrome spinel is enriched in chromium by 10–12 mol % and complementary depleted in aluminum. The composition of plagioclase is enriched in the anorthite component by 3 mol % in dacites and 8 mol % in andesites; this effect was not established in basalts, but its strengthening can be expected. Olivine and orthopyroxene demonstrate an insignificant increase in magnesian content, but the field of their coexistence is significantly reduced in favor of orthopyroxene, which should inevitably lead to a change in their cotectic and peritectic proportions. Natural observations that can be interpreted as the result of the sulfur effect are taken from publications. The results obtained lead to the conclusion that sulfur significantly affects the crystallization of silicate magmas. It follows that the petrological dependences calibrated for sulfur-free experiments should be applied with caution to the interpretation of sulfur-bearing systems. [ABSTRACT FROM AUTHOR]

Sample return missions have provided the basis for understanding the thermochemical evolution of the Moon. Mare basalt sources are likely to have originated from partial melting of lunar magma ocean cumulates after solidification from an initially molten state. Some of the Apollo mare basalts show evidence for the presence in their source of a late-stage radiogenic heat-producing incompatible element-rich layer, known for its enrichment in potassium, rare-earth elements, and phosphorus (KREEP). Here we show the most depleted lunar meteorite, Asuka-881757, and associated mare basalts, represent ancient (~3.9 Ga) partial melts of KREEP-free Fe-rich mantle. Petrological modeling demonstrates that these basalts were generated at lower temperatures and shallower depths than typical Apollo mare basalts. Calculated mantle potential temperatures of these rocks suggest a relatively cooler mantle source and lower surface heat flow than those associated with later-erupted mare basalts, suggesting a fundamental shift in melting regime in the Moon from ~3.9 to ~3.3 Ga. Ancient (~3.9 Ga) KREEP-free basalts were sourced from a relatively cool and shallow pyroxene-rich mantle distinct from later-erupted (<3.8 Ga) KREEP-bearing basalts, indicating a fundamental change in melting regimes in the Moon. [ABSTRACT FROM AUTHOR]

The Oulanka group is a compact group of three peridotite–gabbronorite intrusions that is convenient for testing various petrogenetic concepts. The three intrusions are similar in age and occur not far from one another but differ in the composition of their original magmas, are characterized by different sets of cumulus mineral assemblages, and are different in inner structure and rhythmic layering. We applied cluster analysis of the contents of major elements to reproduce the cumulus mineral assemblages of the isochemically altered rocks of the Tsipringa and Lukkulaisvaara massifs. Although the parental magmas of the Kivakka and Tsipringa massifs were of different composition and their crystallization sequences were also different, the vertical sections of these massifs can be clearly subdivided into zones according to their cumulus mineral assemblages, with the limited development of rhythmic interbedding (with individual rhythms ranging from a few to a few dozen meters in thickness). Conversely, the Lukkulaisvaara intrusion does not possess any clearly distinguishable cumulus zones, and large-scale rhythmic layering is traceable throughout the entire thickness of the massif (with rhythms ranging from a few dozen to a few hundred meters in thickness). The different character of the rhythmic layering of the three intrusions may provide an insight into the different scenarios of magma convection in the chambers. [ABSTRACT FROM AUTHOR]