Crystal Mush Research Articles

Basaltic thermal and material inputs in rhyolite petrogenesis: the Chhapariyali rhyolite dyke, Saurashtra, Deccan Traps

The Saurashtra peninsula in the northwestern Deccan Traps continental flood basalt province, India, contains notable concentrations of rhyolitic rocks. The Chhapariyali rhyolite dyke, part of the compositionally diverse Southeastern Saurashtra dyke swarm, intrudes basaltic lava flows. It shows vitrophyric portions, basaltic magma enclaves, gabbroic mineral assemblages (plagioclase + clinopyroxene ± Fe-Ti oxide aggregates), spectacular quench textures, and intense spherulitisation. Thermobarometric calculations on equilibrium mineral–whole-rock (or glass) pairs indicate clinopyroxene crystallisation at 1120–890 °C overlapping with plagioclase crystallisation at 948–932 °C, and a pressure range of 3.6–0.1 kbar, indicating crystallisation during magma storage or ascent in the upper crust. Petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd isotopic) data suggest advanced fractional crystallisation of a mafic magma with considerable assimilation of ancient granitic crust, or anatexis of such crust, due to heating by a basaltic magma chamber. In either scenario, abundant gabbroic cumulates were left by the crystallising basaltic magmas. A new basalt magma recharging the chamber entrained the gabbroic crystal mush and formed enclaves by mingling with the rhyolite magma stored above. Continued basaltic recharge pushed the enclave-bearing, gabbroic cargo-laden rhyolite magma out of the chamber and into the basaltic lava flows via a dyke. Strong supercooling within cold basaltic rock, possibly aided by meteoric water ingress, led to the development of spectacular quench textures in the crystallising rhyolite, whereas extensive low-temperature alteration produced intense spherulitisation. This case study of the Chhapariyali dyke underscores significant thermal and material inputs from basalt in rhyolite petrogenesis.

A frozen oceanic crystal mush

AbstractThe processes driving the evolution of crystal mushes are often documented in complex systems where crystallization, assimilation, magma replenishment and mixing occur concurrently and are generally overprinted by compaction and deformation. Documenting the characteristics of an undisturbed crystal mush is thus of upmost importance; it highlights the initial conditions with which complex crystal mush processes proceed. We here present the structure and composition of an oceanic crystal mush through detailed petro‐structural and chemical study of metre‐scale intrusions from the Mid‐Atlantic Ridge. Textures, bulk‐rock and mineral compositions indicate closed‐system crystallization of primitive melts, undisturbed by dissolution–precipitation reactions and subsequent deformation. These frozen crystal mushes record the simplest possible evolution of small‐scale intrusions and can be used as a baseline to pinpoint the impact of crystal mush processes on the evolution of complex systems. Any divergence from this reference results from processes occurring concomitantly to the progressive closure of the magmatic system.

A nearly isobaric P-T-deformation evolution path of the Cerro Negro Paleoproterozoic mylonitic igneous suites: The exhumation of the Tandilia Belt of the Rio de la Plata Craton

Two high-grade mylonites from Cerro Negro hill were studied in detail to shed light on the igneous-metamorphic evolution of the northwestern extreme of the Azul Megashear Zone (AMSZ) of the Tandilia Belt, Rio de la Plata craton. Bulk-rock compositions of a garnet-bearing hornblende gneissic mylonite (ribbon mylonite) and a xenolith of garnet-bearing granite mylonite (striped gneiss) included in the former were modeled with the GeoPs software (Xiang and Connolly, 2021) to construct P-T pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H by Gibbs energy minimization. Pseudosections were contoured for isopleths parameters and mineral isomodes of interest. Garnet, with a homogeneous composition of pyr5(gro + andr)18.5spes6.5alm70, present in the younger granite crystal mush (<8 vol% of melt for 0.6 wt% of H2O) formed at 820 °C and 7.2 kbar. These conditions were encompassed by biotite with 0.21–0.23 Ti pfu and amphibole with Mg# of 0.30–0.33. A nearly isobaric cooling is outlined during a high-grade deformational event (D1) until core with the composition of pyr5(gro + andr)10spes5alm80, in slightly zoned garnet of the granite xenolith (modeled with 0.3 wt% of H2O), yields conditions of 540–580 °C and 7.2–8.2 kbars. Rimwards, a subsequent cooling with exhumation was recorded in garnet composition of pyr3(gro + andr)17spes6alm72 to reach conditions of ∼420–440 °C and ∼6.6 kbar. The integrated P-T path holds conditions of middle crust realms (ca. 24–25 km) from the initiation of deformation (D1) coevally to, or shortly, after the younger granite crystallization with a waning apparent geothermal gradient to 20 C°Km−1. Consistent mechanisms of deformation in minerals (d) generated a subvertical E-W mylonitic foliation (D1) under granulite to amphibolite metamorphic facies conditions until greenschist metamorphic facies conditions (D2). Fragile events (D3 and D4) occurred at higher crustal levels concomitant with water influx and sporadic retrogression. Hitherto, the studied rocks experienced a somewhat different evolution path when compared to adjacent Paleoproterozoic areas around the AMSZ or from other places of the Tandilia Belt, which show gently partial exhumation P-T paths after their solidus.

Transport of melt, pressure and heat through a magma mush

Prior to intrusion, magma migrates through the crustal plumbing system that likely contains layers or columns of crystal mush. To better understand the behavior of the crustal magmatic system during magmatic unrest, it is important to examine the process of melt migration within the crystal mush and the associated evolution in pressure and temperature. In this study I use an analytical model to explore the characteristics of transport of melt, pressure, and heat through an idealized crystal mush layer/column under uniaxial strain condition. The model invokes a thermo-poro-viscoelastic rheology and uses a frequency-domain method to explore two scenarios of magmatic unrest: harmonic perturbation of fluid pressure, and step-rise in fluid pressure at a source location. Several factors influence the transport of melt, pressure and heat, including the thermal-mechanical coupling arising from the mush rheology, the advection of heat by melt flows, the competition between thermal diffusivity and poroelastic diffusivity, and the viscoelastic relaxation of the crystalline framework. One key finding is the development of transport asymmetry: when a background temperature gradient exists, the transport properties become different for propagation along the background thermal gradient and propagation against the background thermal gradient. Analysis on an endmember case shows that the transport asymmetry is associated to the competition between the diffusion and advection of pore pressure, which determines a Peclet number that depends on the temperature difference across the mush and the thermal expansion coefficients. Because the temperature in magma mushes in the crust likely increase with depth, the observed propagation asymmetry suggests some intrinsic difference between a bottom-up vs. a top-down triggering mechanism for magmatic unrest. The results from this study highlight the importance for further exploration for a more complete description of the transport properties in the crystal mush.

Mush Amalgamation, Short Residence, and Sparse Detectability of Eruptible Magma Before Andean Super‐Eruptions

AbstractGiant 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.

Origin of K-feldspar megacrysts of Quxu batholith in Gangdese belt, South Tibet: Implication for magma rejuvenation in a crystal mush reservoir

Origin of K-feldspar megacrysts of Quxu batholith in Gangdese belt, South Tibet: Implication for magma rejuvenation in a crystal mush reservoir

Petrogenesis of thick, high-grade PGE mineralisation in the Flatreef, northern Bushveld Complex

The Flatreef of the northern limb of the Bushveld Complex is a world-class platinum-group element (PGE) deposit representing the down-dip extension of the Platreef. Exploration drilling in the central portion of the project area intersected a particularly thick PGE-mineralised portion of the Flatreef exposed in drill core TMT006 and its two deflections, containing 4.5 ppm PGE (Pt + Pd + Rh + Au) over 90 m and ~ 3.7 ppm PGE over 150 m. Data obtained using whole rock geochemical analysis, field emission scanning electron microscopy, micro X-ray fluorescence and X-ray computed tomography indicate that the PGE are hosted by, or are spatially closely associated with, disseminated sulfides that occur interstitial to the cumulus silicate minerals. PGE grades show a broad, albeit irregular trend of progressive downward decrease, whereas most metal ratios (Cu/Ni, Pt/Rh, Cu/Pd and, to a lesser degree, Pt/Pd) remain broadly constant throughout the 90 m main ore zone. These features are interpreted to reflect two stages of ore formation: (i) hydrodynamic concentration of sulfidic ultramafic crystal mushes and (ii) localised downward percolation of sulfide melt in response to fluidisation of the cumulate mush caused by infiltration of ascending magmatic or crustal fluids. The model is consistent with the close spatial association of the sulfides with interstitial plagioclase as well as accessory phlogopite, quartz, apatite and rutile, whereas cumulus orthopyroxene is largely free of sulfide.

Crystallization and Solidification of Poikilitic and Granular Rocks in the Ultramafic Sequence of the Xinjie Layered Intrusion (SW China): Constraints from Complex Growth Zoning of Clinopyroxene and Spatial Variation of Dihedral Angles

AbstractCrystallization and solidification pathways in crustal magma chambers control the chemical and textural evolution of igneous rocks. However, deciphering these codes from completely solidified cumulates is challenging and requires a full understanding of growth processes of minerals and textures of rocks. In this study, we collected samples from a ca. 380-m-thick drill core intersecting the ultramafic sequence of the Xinjie layered intrusion in SW China, which is composed of wehrlite with poikilitic clinopyroxene (Cpx), and clinopyroxenite and olivine clinopyroxenite with euhedral to subhedral Cpx grains. Using micro-X-ray fluorescence and electron probe micro-analyzer mapping techniques, we found that the Cpx grains in clinopyroxenite and olivine clinopyroxenite are characterized by sector-zoned, Cr-rich cores, whereas the Cpx oikocrysts in wehrlite only show Cr-rich cores without sector zoning. The sector-zoned cores of the Cpx grains are widespread in clinopyroxenite and olivine clinopyroxenite and consist of Si–Mg-rich hourglass sectors and Cr–Al–Ti-rich prism sectors, indicating rapid growth of the Cpx grains due to moderate degrees of undercooling, which may be induced by fast cooling during initial emplacement of Cpx-saturated magma into cold country rocks. The sector-zoned Cpx grains thus likely constructed the initial crystal framework in the magma chamber. In contrast, the absence of sector zoning in Cr-rich cores of the Cpx oikocrysts indicates that an olivine-saturated magma may have intruded a high-temperature, Cpx-laden crystal mush so that magma undercooling was significantly reduced or eliminated and no sector zoning developed in the Cpx oikocrysts. Such a process may also result in the spatial variation of the medians of Cpx–Cpx–plagioclase dihedral angles (Θccp) in clinopyroxenite, which shows a significant reduction from an original impingement angle of ~60° in the middle and upper parts to ~40° in the basal part of the sequence. The reduction of Θccp could be attributed to melt-present textural equilibration in the Cpx-laden mush that was sustained by additional heat supplied by massive olivine-saturated melt input in the ~40-m-thick basal part. Here, we propose that the ultramafic sequence of the Xinjie layered intrusion may have formed by the intrusion of primitive, olivine-saturated magma into an early-formed Cpx-laden mush in a manner of sill-like sheets. This study provides new perspectives on the early history of a magma chamber and emphasizes the role of sill intrusion of primitive magmas into crystal mush in the formation of ultramafic sequences in layered intrusions, which has been relatively overlooked in previous studies.

Cumulate Formation and Melt Extraction from Mush-Dominated Magma Reservoirs: The Melt Flush Process Exemplified at Mid-Ocean Ridges

Abstract 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.

Partial melting of amphibolitic lower crust and subsequent melt-crystal separation for generation of the Early Eocene magmatism in eastern Himalaya

The Himalayan leucogranites provide a good opportunity to investigate the crustal evolution of the southern Qinghai-Tibet Plateau. In this study, we present zircon U-Pb and monazite U-Th-Pb ages, zircon Hf isotopes and whole-rock Sr-Nd-Pb isotopes and major and trace elements for the Liemai two-mica granite, eastern Himalaya. Together with previously published data we revalued the petrogenesis of the Early Eocene magmatic rocks in this region and their geological implications. The zircon and monazite U-(Th)-Pb dating results showed that the Liemai two-mica granite was generated at ∼ 43 Ma, similar to adjacent Yardoi, Dala and Quedang adakitic two-mica granites, Ridang subvolcanic rocks and Yardoi leucogranite. The Liemai two-mica granite, similar to these coeval adakitic two-mica granites, is enriched in SiO2, Al2O3, Th, U, Pb, La, and Sr, and depleted in MgO, total iron, Yb and Y with high Sr/Y and (La/Yb)N ratios (showing adakitic affinities), and exhibits enriched Sr-Nd-Pb-Hf isotopic compositions, suggesting an origin of a thickened lower crust consisting mainly of garnet amphibolite. Although the Ridang subvolcanic rocks and Yardoi leucogranite show similar Sr-Nd-Hf isotopes to these adjacent coeval two-mica granites, perceptible differences in whole-rock major and trace elements can be observed. Broadly, these granites can be divided into high-Mg# granites (HMGs, the two-mica granites) and low-Mg# granites (LMGs, the Ridang subvolcanic rocks and Yardoi leucogranite). The former has relatively higher contents of total iron, MgO, Mg#, TiO2, P2O5, LREE, Y, Th, Sr, incompatible elements (Cr and Ni) and Eu/Eu* values, and lower contents of SiO2 and Rb/Sr and Rb/Ba ratios, thus is less evolved than the latter. According to recent studies of differentiation processes in silicic magma reservoirs, we proposed that the HMGs represent a congealed crystal mush that was composed of ‘cumulate crystals’ and a trapped interstitial liquid, while the LMGs represent the almost pure liquid that was extracted from the crystal mush. Modeling using the trace elements Sr and Ba shows that the extraction probably occurred when the crystallinity of the mush was ∼ 60%–63%, at least for the most evolved LMGs sample. The HMGs correspond to a residual crystal mush that had a terminal porosity of ∼ 21%–25% filled with a trapped interstitial liquid. Underplating of mafic magmas following slab breakoff of the Neo-Tethys oceanic lithosphere caused partial melting of the amphibolitic lower crust, which had been thickened to ~50 km prior to ~43 Ma.

Contrasting magmatic evolutions of the Three Sister Volcanoes reflect increased heat flow, crustal melting and silicic magmatism in the Central Oregon Cascade Arc

The Three Sisters stretch, one of the most magmatically active segments of the Oregon Cascade Arc, includes the Three Sisters volcanoes proper (North, Middle and South) as well as the shields and cinder cones of the Mount Bachelor Chain and McKenzie Pass. The Three Sisters record a magmatic transition from fundamentally basaltic andesitic activity in older North Sister products to more diverse activity at Middle Sister, and unusually silicic activity in a Cascade volcano, including rhyolite lava, at South Sister.North Sister magmas reflect magmatic evolution of basaltic andesite through crystal fractionation, mostly of olivine, and incorporation of crustal melts. Middle Sister, although mostly composed of basaltic andesite and andesite, produced dacitic magma through assimilation fractional crystallization processes involving incorporation of silicic melts. South Sister developed a subjacent magma chamber under the southern half of the volcano, where andesitic magmas evolved, dominantly through fractional crystallization, to dacite and rhyolite. A prominent compositional gap at South Sister between dacite and rhyolite and depletion of middle and heavy rare earth elements in rhyolite suggest that amphibole was prominently involved in fractionation processes involving separation of silicic magma from a gabbroic or amphibolitic crystal mush. Silicic magma eventually formed a stagnant cap in the system, where it evolved through small scale fractional crystallization. Rhyolite magma was finally heated to elevated temperatures prior to eruption, presumably through influx of less evolved magma into the base of the stagnant silicic cap and was erupted in late rhyolite lava of Rock Mesa and the Devil's Hill Chain.South Sister lies along strike of the Northwest Rift of Newberry Volcano, which forms the northwestern terminus of the High Lava Plains trend of south-central Oregon. The increased magmatic activity of the Three Sisters stretch and unusual abundance of silicic products, including rhyolite, of South Sister may reflect the arrival and intersection of the High Plains magmatic anomaly with the Cascade Arc.

Magma recharge and mush rejuvenation drive paroxysmal activity at Stromboli volcano

Open-conduit basaltic volcanoes can be characterised by sudden large explosive events (paroxysms) that interrupt normal effusive and mild explosive activity. In June-August 2019, one major explosion and two paroxysms occurred at Stromboli volcano (Italy) within only 64 days. Here, via a multifaceted approach using clinopyroxene, we show arrival of mafic recharges up to a few days before the onset of these events and their effects on the eruption pattern at Stromboli, as a prime example of a persistently active, open-conduit basaltic volcano. Our data indicate a rejuvenated Stromboli plumbing system where the extant crystal mush is efficiently permeated by recharge magmas with minimum remobilisation promoting a direct linkage between the deeper and the shallow reservoirs that sustains the currently observed larger variability of eruptive behaviour. Our approach provides vital insights into magma dynamics and their effects on monitoring signals demonstrating the power of petrological studies in interpreting patterns of surficial activity.

ICDP Oman Drilling Project: varitextured gabbros from the dike–gabbro transition within drill core GT3A

Abstract. The Oman ophiolite (Samail massif, Sultanate of Oman) is the largest sub-aerial exposure of oceanic lithosphere on Earth and provides the opportunity to study the accretion and alteration of oceanic lithosphere formed under fast-spreading conditions. Drill hole GT3A (23∘06′50.7′′ N, 58∘12′42.2′′ E) of the ICDP (International Continental Scientific Drilling Program) Oman Drilling Project with a length of 400 m aimed at penetrating the dike–gabbro transition of the Samail ophiolite paleocrust in order to shed light on the role of the axial melt lens (AML) during accretion of the lower plutonic crust. AMLs beneath fast-spreading mid-ocean ridges are sandwiched between the sheeted dike complex and the uppermost gabbros and are believed to feed the upper crust and, at least partially, the underlying crystal mush. Typical gabbroic rocks from dike–gabbro transitions of fast-spreading systems are the so-called “varitextured gabbros”, often showing considerable variations in mineral mode, texture and grain size, which are regarded as the frozen fillings of axial melt lenses. Here, we present a detailed petrographic, microanalytical and bulk-chemical investigation of 36 mafic rocks from the drill hole GT3A, which represent mostly varitextured gabbros, revealing a complex formation with several evolution stages. Poikilitic domains formed first, corresponding to an early crystallization stage, where only plagioclase and clinopyroxene of more primitive composition crystallized. Later, domains of granular textures containing also interstitial amphibole and Fe–Ti oxide were formed. This stage is characterized by a magma evolution that underwent crystal fractionation established by lower temperatures due to more efficient hydrothermal cooling at the margin of the AML. A last stage is characterized by pervasive hydrothermal alteration, where all primary minerals have been altered under temperature conditions, varying from the magmatic regime down to greenschist facies. A highlight of this stage is amphiboles showing noticeable compositional zoning. The observation of peculiar microgranular domains, representing relics of stoped exogenic material from the sheeted dike complex, documents the upward migration of an AML in a replenishment event, forcing the AML to burn through previously altered sheeted dikes. This process is responsible for significant assimilation of hydrothermally altered components, indicated by a marked Cl enrichment in the outer zones of magmatic amphiboles. Petrological modeling involving gabbros and basalts revealed that the GT3A rock suite followed a fractional crystallization evolution trend, with a primitive MORB as parental melt with an estimated water content of 0.2 wt % to 0.8 wt %. The modeled liquid lines of descent suggest a magmatic evolution via fractional crystallization, where the basalts correspond to frozen liquids, while the gabbros, especially the more primitive ones, show a significant cumulate component.

Magma accumulation at depths of prior rhyolite storage beneath Yellowstone Caldera.

Seismic tomography has provided key insight into Yellowstone's crustal magmatic system that includes attempts to understand the melt distribution in the subsurface and the current stage of the volcano's life cycle. We present new tomographic images of the shear wave speed of the Yellowstone magmatic system based on full waveform inversion of ambient noise correlations, which illuminates shear wave speed reductions of greater than 30% associated with Yellowstone's silicic magma reservoir. The slowest seismic wave speeds (shear wave speed less than 2.3 kilometers per second) are present at depths between 3 and 8 kilometers, overlapping with petrological estimates of the assembly depth of erupted rhyolite bodies. Assuming that Yellowstone's magmatic system is a crystal mush with broadly distributed melt, we estimate a partial melt fraction of 16 to 20%.

Plutonic-subvolcanic connection of the Himalayan leucogranites: Insights from the Eocene Lhunze complex, southern Tibet

The Lhunze region of southeast Tibet uniquely juxtaposes Cenozoic granitic plutons with subvolcanic rocks in the Tethyan Himalayan Sequence. In this study, the potential petrogenetic connection between two-mica granite and subvolcanic rhyolite porphyry from the Lhunze area is investigated using petrology, geochronology, and geochemistry. Monazite Th Pb ages imply the two-mica granite crystallized during the mid-Eocene at c. 43.5–43.1 Ma, consistent with previous zircon U Pb isotope data. Monazite in the porphyry also records Th Pb ages of c. 44.5–42.9 Ma , suggesting contemporaneous emplacement with the two-mica granite. Porphyritic structure, feldspar aggregates and comb alignment demonstrate that the two-mica granite resulted from transient melt flow associated with melt extraction. Some plagioclase phenocrysts from the porphyry have consistent An contents with the two-mica granite, indicating that both the two-mica granite and the porphyry probably originated from a similar melt. However, the porphyry has a higher SiO 2 concentration (72.2–75.9 wt%) than the two-mica granite (69.0–73.0 wt%). In addition, the porphyry is characterized by lower TiO 2 , MgO, CaO, Ba, Sr, and Zr contents as well as Zr/Hf, Sr/Y, Nb/Ta ratios than the two-mica granite. Moreover, strontium and REE contents of monazite in both the two-mica granite and porphyry also imply that crystal fractionation dominated melt evolution. Whole-rock Sr Nd and in situ monazite Nd isotope compositions indicate that the porphyry is slightly more radiogenic than the two-mica granite, implying that the porphyry likely assimilated more ancient country rocks during its evolution. We interpret the two-mica granite and porphyry as comagmatic intrusives linked by fractional crystallization of a cognate magma mush, in which the crystals accumulated to form the two-mica granite, and the complementary high silicic melt segregated to form the porphyry. Plagioclase accumulation is responsible for the generation of the high Sr /Y adakitic geochemical feather within two-mica granite. The scenario of complementary melt extraction and crystal fractionation might also have been an important process in forming the Himalayan leucogranites. Regionally, Neo-Tethyan slab breakoff during the Eocene could have increased mantle heat flow and triggered the generation and extraction of large volumes of silicic melt. • Two-mica granite and subvolcanic rhyolite porphyry have a coeval crystallization age at 44–43 Ma . • Two-mica granite represents cumulate and the porphyry as its complementary highly fractionated melt. • Magma differentiation via melt extraction from crystal mush is an effective mechanism to produce Himalayan leucogranites.

Precaldera Mafic Magmatism at Long Valley, California: Magma-Tectonic Siting and Incubation of the Great Rhyolite System

The iconic volcanic center at Long Valley has released ∼820 km3 of rhyolite in at least 110 eruptions. From 2.2 Ma until 0.23 Ma, products were exclusively rhyolitic, and ∼ 700 km3 were high-silica rhyolite severely depleted in Sr, Ba, and Eu. The rhyolitic interval was preceded by an interval from 3.9 to 2.6 Ma with numerous basalt-andesite-dacite eruptions accompanied by no rhyolite at all. We have now mapped the circumcaldera products of this interval, defined 107 eruptive units, characterized them all chemically and petrographically, and dated many by 40Ar/39Ar. Here we display and describe them by sector around the caldera, interpret the nature of the transcrustal magma system that eventuated in the 35-km-wide Long Valley granite-rhyolite pluton, and analyze regional tectonic factors that did or did not contribute to siting the system.Nine Miocene (12–6 Ma) eruptive units close to Long Valley were followed by a Pliocene flare-up that released >300 mafic eruptions in a SW–NE swath 170 km long, centered across the later site of Long Valley. The basalts and their fractionates are intraplate alkalic products dominated by a continental lithosphere that had long been fluxed by Mesozoic subduction. Tertiary arc volcanism had not impinged on the area of Long Valley. Volumes estimated for the 107 Neogene precaldera eruptive units (only 40 of which exceeded 0.1 km3) total ∼ 27 km3 ± 50%—only ∼3% of the subsequent volume of rhyolite erupted. Such a volume of high-silica rhyolite with ultra-low Sr and Eu is not a product of partial melting but requires as proximate parent a leucogranitic crystal mush that is itself the upper level of a long-lived plutonic reservoir that extends to the lower crust. The 27 km3 of Neogene magma that erupted was a small contingent of the mantle-derived basaltic flux needed to energize (and contribute its fractionated melt to) a 30-km-deep compositionally graded crustal column, which culminated in ∼10,000 km3 of granitoid mush from which 820 km3 of Quaternary high-silica rhyolitic melt escaped and erupted.Pliocene basaltic eruptions ceased at ∼2.6 Ma, probably because the basaltic flux intensified sufficiently to render the mushy upper crust impenetrable. The 2.6–2.2 Ma quiescent interval represented culmination of thermal activation of the plutonic column and refinement of its leucogranitic mushy upper layer, from which extreme melts escaped for the next 2 Myr. The Pliocene mafic swath crosses the Sierran rangefront fault zone coincident with a left-stepping extensional reentrant that also began developing at ∼3 Ma. Moreover, Long Valley overlies a dextral offset in the initial Sr-isotope 0.706 line, which may reflect the rifted or attenuated edge of Proterozoic crust and mantle lithosphere. Concatenation of these three influences may account for siting of intensified edge-focused magmatism that produced the great Quaternary pluton.

Petrogenesis of the Newly Discovered Early Cretaceous Peralkaline Granitic Dikes in Baerzhe Area of Jarud Banner, Inner Mongolia: Implications for Deciphering Magma Evolution

The super-large Baerzhe Be–Nb–Zr–REE deposit in NE China is hosted in the Early Cretaceous peralkaline granites. In this work, the newly discovered granitic dikes developed around the Baerzhe deposit were studied for the first time, focusing on their genesis and genetic relationships with the Baerzhe peralkaline granites. Zircon U-Pb dating of these granitic rocks (including the granite porphyry, rhyolite and miarolitic granite) yielded Early Cretaceous ages of 125–121 Ma. Their mineral assemblages and geochemical features suggest that they share similar features with the peralkaline A-type granites. Their geochemical data and zircon Hf isotopic compositions (εHf(t) = +3.4 to +10.5) indicate that the peralkaline granitic rocks were formed by the partial melting of dehydrated charnockite with extensive plagioclase crystal fractionation, which resulted in a peralkaline affinity. There are two types of distinct zircons in the studied samples: the type I zircon with a bright rim and dark core, which may represent a cumulate mineral phase captured together with aggregates during eruption, and the type II zircon with a higher evolution degree crystallized in the residual melts. Combined with the simulation results using whole-rock trace elements, we proposed that the peralkaline granitic dikes represent more evolved interstitial melts than the Baerzhe granitic magma. In the Early Cretaceous extensional tectonic settings, mantle-derived magma upwelled, which induced the melting of the lower crust and prolonged the evolutionary process of the magma crystal mush.

A crystal mush origin for the high Sr/Y plutons: Insights from the Nyemo Igneous Complex in the Gangdese arc, southern Tibet

High Sr/Y plutons generated in continental arc resemble geochemically the Archean trondhjemite-tonalite-granodiorite suite and are widely used to interpret processes of intermediate-felsic crust generation during the Archean. However, this geochemical signature could be overinterpreted without detailed petrographic observation. Here we carry out a study of detailed field and petrographic observations, geochronology , and geochemistry for the Nyemo Igneous Complex (NIC) in the central Gangdese arc, southern Tibet . Our results show that the NIC is composed of gabbros , monzogabbros, monzodiorites, monzonites , and monzogranites that were formed at 91–89 Ma. Petrographic observations and geochemical data suggest that the diverse lithologies of the NIC can be ascribed to a cogenetic magma fractionation sequence defined by olivine → orthopyroxene + clinopyroxene + apatite + Fe-Ti oxides → plagioclase + biotite → K-feldspar → amphibole . The early crystallization of biotite and relatively late crystallization of amphibole indicates that the primary melts of the NIC have low water contents. The monzodiorites and monzogranites from the NIC have high Sr (412–1026 ppm), low Y (4.30–25.4 ppm), and thus high Sr/Y ratios (16–140). The orthocumulate texture defined by euhedral phenocrysts of normally zoned plagioclase and subhedral biotite indicates these high Sr/Y rocks are unlikely pure melts and instead represent fossil plagioclase-rich magma mush. We emphasize that the high Sr/Y signature does not necessarily reflect the pressures of magma generation involved of garnet/amphibole as residual or fractional phases. Late Cretaceous arc magmatic rocks in the eastern and central Gangdese arc show different crystallization sequences, requiring the presence of contemporaneous primary melts with distinct H 2 O contents along the Gangdese arc, which probably were caused by variable water input from the Neo-Tethyan oceanic lithosphere . • Nyemo Igneous Complex (NIC) in the Gangdese arc represents a fractionation sequence. • The primary melts of the Nyemo Igneous Complex have low water contents. • High Sr/Y plutons in the NIC represent fossil plagioclase-rich magma mush. • Arc magma evolution is controlled by the water input from the oceanic lithosphere.

The Magmatic Evolution and the Regional Context of the 1835 AD Osorno Volcano Products (41°06’S, Southern Chile)

Abstract Osorno volcano (41°06’S, 72°20’W) is a composite stratovolcano of the Central Southern Volcanic Zone of the Chilean Andes. It is the southernmost member of a NE–SW trending alignment of volcanic edifices including La Picada and Puntiagudo volcanoes and the Cordón Cenizos chain. According to contemporary descriptions recorded by Charles Darwin in 1835, two eruptive events occurred: the first during January–February, and the second during November–December 1835 and January 1836. The volcano erupted basaltic andesite lavas and tephra fall deposits (52.4 to 52.9 SiO2 wt. %), which contain phenocrysts of olivine, plagioclase, clinopyroxene, and spinel. The compositions of these phenocryst phases, together with those of olivine-hosted melt inclusions, allowed us to constrain intensive parameters for the pre-eruptive magmas. These varied from 1060°C to 1140°C, with an oxygen fugacity buffer of ~ΔQFM +1.1, dissolved water concentrations of up to 5.6 wt. % (average of ~4.2 wt. %) and maximum pressures equivalent to ~7-km depth. Textural relations, such as crystal accumulations and clots, zoning in crystals and other indications of disequilibrium, lead us to infer the involvement of a crystal mush, rich in individual crystals and clots of crystals, which underwent a degree of disaggregation and entrainment into the transiting magma prior to eruption. Comparison of trace element abundances, including rare earth elements, fluid-mobile elements, and relatively fluid-immobile elements, combined with 87Sr/86Sr and 143Nd/144Nd isotope ratios, allows us to consider variations in slab-derived fluid input and the minor role of crustal contamination on the Osorno eruptive products and those from neighboring volcanic systems. Our results suggest both a greater contribution from slab-derived fluid and a higher degree of partial melting in the systems supplying stratovolcanoes (Osorno, Calbuco, and La Picada) relative to those supplying small eruptive centers built over the major regional Liquiñe-Ofqui Fault Zone.

Advances in seismic imaging of magma and crystal mush

Seismic imaging methods have provided detailed three-dimensional constraints on the physical properties of magmatic systems leading to invaluable insight into the storage, differentiation and dynamics of magma. These constraints have been crucial to the development of our modern understanding of magmatic systems. However, there are still outstanding knowledge gaps resulting from the challenges inherent in seismic imaging of volcanoes. These challenges stem from the complex physics of wave propagation across highly heterogeneous low-velocity anomalies associated with magma reservoirs. Ray-based seismic imaging methods such as travel-time and surface-wave tomography lead to under-recovery of such velocity anomalies and to under-estimation of melt fractions. This review aims to help the volcanologist to fully utilize the insights gained from seismic imaging and account for the resolution limits. We summarize the advantages and limitations of the most common imaging methods and propose best practices for their implementation and the quantitative interpretation of low-velocity anomalies. We constructed and analysed a database of 277 seismic imaging studies at 78 arc, hotspot and continental rift volcanoes. Each study is accompanied by information about the seismic source, part of the wavefield used, imaging method, any detected low-velocity zones, and estimated melt fraction. Thirty nine studies attempted to estimate melt fractions at 22 different volcanoes. Only five studies have found evidence of melt storage at melt fractions above the critical porosity that separates crystal mush from mobile magma. The median reported melt fraction is 13% suggesting that magma storage is dominated by low-melt fraction crystal mush. However, due to the limits of seismic resolution, the seismological evidence does not rule out the presence of small (&amp;lt;10 km3) and medium-sized (&amp;lt;100 km3) high-melt fraction magma chambers at many of the studied volcanoes. The combination of multiple tomographic imaging methods and the wider adoption of methods that use more of the seismic wavefield than the first arriving travel-times, promise to overcome some of the limitations of seismic tomography and provide more reliable constraints on melt fractions. Wider adoption of these new methods and advances in data collection are needed to enable a revolution in imaging magma reservoirs.