Experimental study of high-Ti and low-Ti basalts: liquid lines of descent and silicate liquid immiscibility in large igneous provinces

peer reviewedDichotomous high-Ti and low-Ti magmas are ubiquitous in large igneous provinces (LIPs). These magmas often form economically critical Fe–Ti oxide ores in layered intrusions via mechanisms that remain debated. To constrain the evolution of high-Ti and low-Ti basalts during fractionation, we performed stepwise equilibrium crystallization experiments at atmospheric pressure. We specifically aimed to quantify the influences of starting composition and oxygen fugacity (fO2) on phase stability, phase compositions, and the onset of silicate liquid immiscibility during cooling. Both types of magma crystallize similar phase assemblages at QFM (quartz–fayalite–magnetite thermodynamic equilibrium) and QFM + 2: olivine, clinopyroxene, plagioclase, Fe–Ti oxides, and whitlockite. Tridymite crystallizes late in experiments at QFM + 2. The starting composition exerts a strong influence on phase and melt compositions. High CaO and Al2O3 contents in the melt favor the early crystallization of plagioclase and enhance FeO enrichment before Fe–Ti oxide saturation. fO2 affects the composition and stability of Fe–Ti oxides, and high fO2 conditions may promote melt differentiation into the calc-alkaline field. Silicate liquid immiscibility occurs in both compositional trajectories, producing Fe-rich melt globules in equilibrium with Si-rich melts. Strong iron enrichment is not necessary for immiscibility to develop; unmixing also occurs in Fe depleted compositions. We propose a new parameterization to map the binodal surface in temperature-composition space that successfully fits the two-liquid field in experiments and natural immiscible compositions. Our results indicate that Fe–Ti oxide ores in layered intrusions associated with LIPs form by the segregation of Fe-rich melts and/or the accumulation of early crystallized Fe–Ti oxides during fractionation

SilicH2O: a graphical user interface for processing silicate glass Raman spectra and quantifying H2O

H2O contents of magmas strongly impact the explosivity of volcanic eruptions, as well as their rheological properties and crystallisation behaviour. Accurate analyses of H2O in magmatic liquids are therefore vital for our understanding of the dynamics of magmatic processes and eruptions. Raman spectroscopy provides an accessible, affordable and high spatial resolution technique for estimating H2O contents of magmas that have been quenched to a glass during eruption. However, calculating H2O concentrations from Raman spectra involves manual data processing and results are therefore sensitive to the specific treatment used. SilicH2O is an open-source software program that uniformises and streamlines this process by providing an interactive graphical user-interface. It can be used to: (a) process Raman spectra of silicate glasses, (b) remove any unwanted peaks through interpolation and unmixing, (c) set up H2O calibrations with reference materials and (d) quantify H2O contents of unknown samples

Melting processes and mantle sources of lavas on Mercury

peer reviewe

Experimental study of liquid immiscibility in the Kiruna-type Vergenoeg iron–fluorine deposit, South Africa

In this study we experimentally assess whether the bulk composition of the Kiruna-type iron–fluorine Vergenoeg deposit, South Africa (17 wt.% SiO2 and 55 wt.% FeOtot) could correspond to an immiscible Fe-rich melt paired with its host rhyolite. Synthetic powder of the host rhyolite was mixed with mafic end-members (ore rocks) in variable proportions. Experimental conditions were 1–2 kbar and 1010 C, with a range of H2O and F contents in the starting compositions. Pairs of distinct immiscible liquids occur in experiments saturated with fluorite, under relatively dry conditions, and at oxygen fugacity conditions corresponding to FMQ 1.4 to FMQ+1.8 (FMQ = fayalite-magnetite-quartz solid buffer). The Si-rich immiscible liquids contain 60.9–73.0 wt.% SiO2, 9.1–12.5 wt.% FeOtot, 2.4–4.2 wt.% F, and are enriched in Na2O, K2O and Al2O3. The paired Fe-rich immiscible melts have 41.0–49.5 wt.% SiO2, 20.6–36.1 wt.% FeOtot and 4.5–6.0 wt.% F, and are enriched in MgO, CaO and TiO2. Immiscibility does not develop in experiments performed under water-rich (aH2O > 0.2; a = activity) and/or oxidized (>FMQ+1.8) conditions. In all experiments, solid phases are magnetite, ±fayalite, fluorite and tridymite. Our results indicate that the rocks from the Vergenoeg pipe crystallized in a magma chamber hosting two immiscible silicate melts. Crystallization of the pipe from the Fe-rich melt explains its extreme enrichment in Ca, F and Fe compared to the host rhyolitic rocks. However, its low bulk silica content compared to experimental Fe-rich melts indicates that the pipe formed by remobilization of a mafic crystal mush dominated by magnetite and fayalite. Segregation of evolved residual liquids as well as the conjugate immiscible Si-rich melt produced the host rhyolite. The huge amount of fluorine in Vergenoeg ores ( 12 wt.% F) can hardly be explained by simple crystallization of fluorite from the Fe-rich silicate melt (up to 6 wt.% F at fluorite saturation). Instead, we confirm a previous hypothesis that the fluorite enrichment is, in part, due to the migration of hydrothermal fluids

Ubiquitous dendritic olivine constructs initial crystal framework of mafic magma chamber

peer reviewedLayered intrusions are fossilized mafic magma chambers in the Earth's crust. The pathways that led to crystallization and solidification of layered intrusions have been hotly debated as the growth model of primocrysts (the earliest-formed crystals) in mafic magma chambers remains enigmatic. In this study, we carried out high-resolution elemental mapping of mm-scale olivine primocrysts from the Sept Iles layered intrusion (Canada), the third largest one in the world, with a focus on phosphorus (P) zoning of olivine. Our results reveal that complex P zoning of olivine with intense dissolution textures is ubiquitous in the ∼4.7 km-thick Layered Series of the intrusion. The P-rich zones of olivine are featured with dendritic, hopper and sector-zoned patterns, which are attributed to significant magma undercooling. Thermal modeling based on a 1-D conductive cooling model suggests that initially hot parental magma intruding into cold country rocks would result in high degrees of undercooling (-ΔT >60 °C) in the margins (i.e., floor, roof and sidewalls) of magma chamber, facilitating rapid growth of dendritic olivine, which may be then spread within the magma chamber by dynamic convection and crucial to construct initial crystal framework of a solidifying magma chamber. Additionally, diffusion modeling based on the P gradients in olivine suggests a minimum cooling rate of 2.7 to 3.3×10−3 °C/year in the center of the intrusion, similar to the averaged cooling rate of other layered intrusions (e.g., Bushveld, Stillwater and Skaergaard) reported in previous studies. This indicates that rapid cooling (ca. 10−2 to 10−3 °C/year) at high temperature (>800 °C) may be predominant regardless of the size of magma chambers. Our study demonstrates that P zoning of olivine is powerful in decoding crystallization and thermal histories of mafic-ultramafic intrusions

Influence of volatiles (H 2 O and CO 2 ) on shoshonite phase equilibria

t. Experiments were performed at 500 MPa, 1080 °C and water activities (aH2O) from 0.0 to 1.0, in fluid-present and fluid-absent conditions, with the aim of constraining the effect of volatiles on phase equilibrium assemblages of a shoshonite from Vulcanello (Aeolian Islands, Italy). Experiments were run both under reducing and oxidizing conditions and results show that proportions, shapes and size of crystals vary as a function of the volatile composition (XH2O and XCO2) and volatile content. Clinopyroxene (Cpx) is the main crystallising phase and is compositionally analogous to Cpx crystals found in the natural rock. Plagioclase (Pl) is stable only for water activity lower than 0.1, whereas Fe–Ti oxides are present in all experimental runs, except for those where log f O2 was lower than −9, (∆NNO −0.11) irrespective of the presence of CO2. The addition of CO2 (2.8 wt%) in nominally dry experimental charges substantially reduces the crystallinity by ca. 1/3 compared to volatile free experiments. This result has important consequences upon the physical properties of the magma because it influences its viscosity and, as a consequence, velocity during its travel to the Earth surface

The petrology of a hazardous volcano: Calbuco (Central Southern VolcanicZone, Chile)

peer reviewedThe recurrent explosive eruptions of Calbuco (Andean Southern Volcanic Zone (SVZ)) threat a rapidly expanding tour- istic and economic region of Chile. Providing tighter constraints on its magmatic system is therefore important for better monitoring its activity. Calbuco is also distinguished by hornblende-bearing assemblages that contrast with the anhydrous parageneses of most Central SVZ volcanoes. Here we build on previous work to propose a detailed petrological model of the magmatic system beneath Calbuco. Geochemical data acquired on a hundred samples collected in the four units of the volcano show no secular compositional change indicating a steady magmatic system since ~ 300 ka. A tholeiitic Al2O3-rich (20 wt. %) basalt (Mg# = 0.59) is the parent magma of a differentiation trend straddling the tholeiitic/calc-alkaline fields and displaying a narrow compositional Daly gap. Amphibole crystallization was enabled by the higher H2O content of the basalt (3–3.5 wt. % H2O at 50 wt. % SiO2) compared to neighboring volcanoes. This characteristic is inherited from the primary mantle melt and possibly results from a lower degree of partial melting induced by the mantle wedge thermal structure. Although macrocrysts are not all in chemical equilibrium with their host rocks and were thus presumably unlocked from the zoned crystal mush and transported in the carrier melt, the bulk-rock trend follows both experimental liquid lines of descent and the chemical trend of calculated melts in equilibrium with amphibole (AEMs). These contradictory observations can be reconciled if minerals are transported in near cotectic proportions. The AEMs overlap the Daly gap revealing that the missing liquid compositions were present in the storage region. Geothermobarometers all indicate that the chemical diversity from basalt to dacite was acquired at a shallow depth (210–460 MPa). We suggest that differentiation from the primary magma to the parental basalt took place either in the same storage region or at the MOHO.CDR J.00066.14, PDR T.0079.18, Odysseus grant (ON

Calbuco (Central Southern Volcanic Zone, Chile): a hazardous volcano

CDR J.00066.14, PDR T.0079.18, Odysseus grant (ON

What modulates eruptive styles at Villarrica and Osorno volcanoes (Chile)?

Villarrica and Osorno are two active stratovolcanoes in the Central Southern Volcanic Zone (CSVZ) of the Chilean Andes that share several geochemical characteristics: near-primary, tholeiitic parent magmas (50-53 wt. % SiO2), overlapping major/trace element differentiation trends, and comparable storage conditions [1-4]. Yet, their eruptive styles contrast each other significantly. Villarrica is a steady-state, open-vent stratovolcano with a lava lake since 1985, which produced ~100 low to moderate intensity Strombolian eruptions and lava flows since 1579 CE. Osorno is a closed-vent stratovolcano with 10x less eruptions for the same period. We initially proposed that differences in eruptive style and frequency could be due to a relatively higher degree of crustal permeability under Villarrica than Osorno due to the Liquiñe-Ofqui Fault Zone [5]. Preliminary analyses show that both volcanoes have broadly similar olivine chemistry ranges and multimodal distributions, with minor differences in olivine and melt chemistry/textures between Villarrica (Fo72-87) and Osorno (Fo66-82) [4,5]. Diffusion timescales for both volcanoes are mostly 250 days. This suggests the degree of crustal permeability underneath the volcanoes are likely comparable, prompting consideration of other factors. In this contribution, we evaluate the role of magma supply rate, storage conditions, and slab input in modulating eruptive styles at Osorno and Villarrica based on an updated dataset of magma storage conditions, diffusion timescales, geochemical data compilations, and inferences drawn from published literature. We find that magma storage conditions of both volcanoes are similar to each other at T~1100°C, P~200 MPa, along with comparable input of fluids released from the down-going slab. The multimodality in olivine chemistry, diversity in types of olivine growth zones and textures, timescale ranges, coupled with the relatively high magma supply rate estimates for Villarrica from the literature suggest magma supply rate could modulate eruptive style at Villarrica and Osorno. With this contribution, we aim to further current understanding of subduction zone magmatism and geodynamics, with implications on volcanic hazard reduction. 1. Vergara et al. (2004). J. S. Am. Earth Sci. 17: 227-238. 2. Morgado et al. (2015). JVGR, 306: 1-16. 3. Pizarro et al. (2019). JVGR. 384: 48-63. 4. Bechon et al. (2022). Lithos. 106777. 5. Romero et al. (2022). Bull. Volc. 85 (2)