347 research outputs found
A New Plagioclase-Liquid Hygrometer Specific to Trachytic Systems
No full textWe present a new empirical plagioclase-liquid hygrometer for estimating the amount of H2O dissolved in trachytic magmas. The hygrometer is based on the exchange reaction of anorthite between plagioclase and liquid, and is calibrated using crystallization experiments where the concentration of H2O in quenched glasses has been accurately determined based on Fourier Transform Infrared Spectroscopy (FTIR) analysis. The multiple linear regression of plagioclase-liquid cation fractions and components from experimental data obtained at 150–202 MPa, 850–1020 ◦C, 1.17–7.57 wt. % H2O and ∆NNO + 2.5 buffer, yields to a highly accurate model with uncertainty of only ±0.29 wt. % H2O. The model reliability has been demonstrated using an independent test data set consisting of crystallization experiments from the literature and thermodynamically derived compositions. The fairly good convergence between our model calibration and the test data set excludes systematic H2O overestimates or underestimates caused by miscalibration and data overfitting. The plagioclase-liquid hygrometer from this study has been applied to trachyandesitic (latitic) and trachytic products erupted over the last 1000 years at the La Fossa cone of Vulcano Island (Aeolian Islands, Southern Italy). Results from calculations indicate that the concentration of H2O in the latitic and trachytic melts is comprised between ~2.5 and ~3.5 wt. %. These values are in good agreement with data from melt inclusions and, overall, testify to low-pressure, open-system differentiation of trachytic magmas under strong degassing condition
Crustal Magmatic System Evolution: Anatomy, Architecture, and Physico‐Chemical Processes
No full textDegree of sector zoning in clinopyroxene records dynamic magma recharge and ascent
No full textThe development of sector zoning in clinopyroxene is attributed to the influence of crystallisation kinetics imposed by magma undercooling (ΔT) and may reflect variations in magma cooling histories. Yet, the degree of compositional variations between sectors has not been explored as a potential recorder of crystallisation dynamics.
Here, we investigate the distribution of major, minor, and trace elements between hourglass {111} and prism {h k 0} sectors in clinopyroxene with distinct pre-eruptive histories at Mt. Etna, Italy. We analyse sector-zoned clinopyroxene crystals ranging in size from sub-mm to cm (i.e., microphenocrysts, phenocrysts, and megacrysts), from eruptions fed by the central conduits of the volcano (1669 and 2002–03 flank eruptions) and eruptions fed by eccentric dykes which bypass the central conduits, tapping deeper magma storage regions (1974 and 2002–03 flank eruptions). We focus on Cr-rich mantle zones, which crystallised upon eruption triggering mafic rejuvenation and are ubiquitous across our sample set. With decreasing crystal size (i.e., increasing ΔT), tetrahedral aluminium is more strongly partitioned between prism and hourglass sectors. This promotes the uptake of rare earth elements (REE) and high field strength elements (HFSE) into prism relative to hourglass sectors. Combining relative degrees of sector enrichment with ΔT estimates, we propose magma recharge, mush remobilisation and the onset of magma ascent imposed slightly higher ΔT in 1974 than in 2002–03 eruptions at Mt. Etna. Enhanced ΔT in 1974 could be related to vigorous mixing and rapid transport of magma with limited storage, resulting in crystals of smaller sizes. Crystal size populations vary across eruptions, but crystals within a given population (e.g., phenocrysts) return similar calculated ΔT and REE + HFSE sector enrichments, implying connectivity between magmatic environments in the mush system. We show that the magnitude of sector zoning in clinopyroxene can be employed to explore subtle differences in pre-eruptive dynamics in volcanic systems. As an example, we explore sector enrichment in clinopyroxene phenocrysts from the 2021 eruption at La Palma (Canary Islands) and megacrysts from Roman era activity at Stromboli (Italy). Results highlight the role of dynamic mixing and mush remobilisation before eruption in mafic alkaline settings and suggest changes in magma composition across alkaline systems influence clinopyroxene chemistry but do not influence sector enrichment
Preliminary crust-basalt interaction experiments at 0.8–1.0 GPa: new hybrid melts formation
No full textMelt extraction in mush zones. The case of crystal-rich enclaves at the Sabatini volcanic district (central Italy)
No full textA peculiar feature of the Sabatini Volcanic District (SVD, central Italy) is the occurrence of crystal-poor pumices
and crystal-rich enclaves within the same eruptive host-deposit. The stratigraphic sequence of pumices and
enclaves indicates the tapping of a stratified magma chamber, where a crystal-poor phonolitic magma lay on
top of a more primitive crystal-rich magma. The crystal-rich enclaves are genetically related to the pumices
and record the evolution of a solidification front, in which a more differentiated melt was produced, extracted
and eventually erupted. We collected and analyzed crystal-rich enclaves from one of the largest phonolitic eruptions
at the SVD and used their petrological and geochemical features to reconstruct magma differentiation and
crystal-melt separation in the solidification front. On this basis, three groups of enclaves have been identified:
porphyritic enclaves, holocrystalline enclaves and sanidinites. The mineralogical variability faithfully reproduces
the spatial and temporal evolution expected of a solidification front, from early-to-intermediate crystallization
conditions (porphyritic and holocrystalline type) to the late stage of solidification (sanidinites), in which the
percolation of a more differentiated melt through the crystal mush triggered the instability of the solidification
front. Results from numerical models indicate that gravitational instability is the most efficient mechanism to
explain melt extraction in mush zones of medium-sized (~10 km3), short-lived (~104 years) magma chambers
Influence of calcite decarbonation on the frictional behavior of carbonate-bearing gouge: Implications for the instability of volcanic flanks and fault slip
No full textThe impact of heat, readily provided by magma, circulating hot fluids, or rapid fault slip on carbonate substrata is an important factor in determining the flank stability of volcanoes and the mechanical behavior of faults in areas where carbonate lithologies are prevalent. The mineralogy and mechanical properties of carbonate rocks are demonstrably altered by thermally induced decarbonation. While previous studies have considered the role of decarbonation in bulk strength loss at subvolcanic conditions and dynamic weakening during coseismic fault slip, little is known about the effects of decarbonation on the frictional properties of carbonate rocks during both the nucleation and inter-seismic phases. Here, we present results from experiments performed on a portlandite-rich material, a typical hydrous product of the decarbonation reaction. To evaluate frictional strength, stability, and healing at shallow crustal conditions, we sheared gouge layers of this material under saturated conditions at room temperature and at velocities comparable to those involved in earthquake nucleation. Our data indicate that the reaction of calcite to portlandite results in a distinct change in the mechanical behavior of the gouge. Decarbonated shear zones are (1) frictionally weaker at higher normal stresses, (2) more frictionally unstable, and (3) likely to regain their frictional strength more quickly than unaltered calcite-rich zones. The occurrence of portlandite could be key for interpreting the stability of volcanic flanks that root into carbonate substrata and for seismogenic normal faults located within thick sedimentary sequences, and thus provide a better understanding of the hazards they pose
Towards a new clinopyroxene geothermometer for alkaline, differentiated magmas
No full textIn the last decades several clinopyroxene geothermometers have been proposed with the aim to constrain pre-eruptive conditions of volcanic systems [1]. However, the compositional bounds of the calibration dataset represent a serious limitation in their use. In fact, the more the composition of the investigated natural rocks deviates from the compositions in the calibration dataset, the larger the uncertainty in the estimate of temperature will be.
At the present, clinopyroxene geothermometers are mainly calibrated on magma compositions ranging from basalt to rhyolite, leaving poorly constrained or even unconstrained, alkaline differentiated composition. Moreover, the effect of melt-water content on phase compositions is usually neglected. Given the magnitude of the alkaline, explosive volcanism, these two factors cannot be ignored in future calibrations of geothermometers.
In this study, we present a new clinopyroxene geothermometer specifically calibrated for hydrous, alkaline compositions ranging from phonolite to trachyte. This model is based on a broad dataset consisting of 35 phase equilibria experiments, carried out at 200 MPa, in the temperature range 850-1000°C and at variable XH2O-XCO2 (H2O ranging from 0 to 6 wt.% and CO2 ranging from from 0 to 0.5 wt.%). The equations have been obtained by means of least squares regression analysis of the experimental dataset, yielding a better accuracy of temperature estimate than previous models. Notably, the accuracy of the model largely increases by including the water-melt content parameter in the equations, whereas the presence of CO2, which actually does not affect the composition of clinopyroxene and melt, scarcely influences the temperature estimate
Solidification of a rhyolitic magma beneath the Krafla caldera
No full textA shallow, silica-rich rhyolitic magma body was drilled during the perforation of the Krafla caldera, carried out in the framework of the Iceland Deep Drilling Project (IDDP). Samples of the rhyolitic magma were collected among the cuttings brought to the surface by the drilling fluids. These samples consist of vesiculated glassy fragments containing crystals of titanomagnetite, plagioclase and clinopyroxene. Minerals are in textural and chemical disequilibrium with the rhyolitic melt, as indicated by compositional zoning of plagioclase and exsolution lamellae in clinopyroxene. Additionally, Fe-Mg exchange between clinopyroxene and melt (cpx-meltKDFe-Mg) and Ab-An exchange between plagioclase and melt (plg-meltKDAb-An) show values much lower than those expected at equilibrium conditions. These disequilibrium features make difficult to assess correctly the crystallization path of magma through a classical approach based on the use of geotherbarometers or thermodynamic modelling.
Therefore, in order to elucidate the physico-chemical conditions controlling the final stage of magmatic evolution, we aim to investigate experimentally the origin of the rhyolitic magma, which is still under debate. Indeed, the change of the crystallization conditions recorded by minerals may be addressed either to rapid extraction of the rhyolitic melt from a crystal mush, or to slow cooling of the rhyolitic melt produced in-situ by partial melting of the host felsite rock
Spatial and temporal mush heterogeneity during eruptions recorded in clinopyroxene from the 2021 paroxysms at Mt. Etna, Italy
No full textTextural and compositional zoning of volcanic minerals archives pre-eruptive magma processes. Crystals erupted simultaneously may be sampled from different regions of the plumbing system and hence record variable histories due to complex magma dynamics. In addition, crystals erupted throughout the course of an eruption may record temporal variations in the plumbing system. To resolve mush variability on both spatial and temporal scales, we investigate clinopyroxene erupted during a series of paroxysmal episodes between February–April 2021 at Mt. Etna, Italy. Using a combination of high-resolution geochemical techniques, we observe that Cr enrichments in clinopyroxene mantle zones, grown upon eruption-triggering mafic rejuvenation, exhibit both temporal and spatial (sample-scale) variability. Temporal variability correlates with changes in glass compositions, attesting to the ability of clinopyroxene to track magma maficity throughout an eruption. Spatial variability, indicated by the scatter of Cr concentrations, is greatest for the first event and lowest for the final paroxysm. In conjunction with core textures, degree of sector enrichment and thermobarometry, our data suggest that the onset of the paroxysms was preceded by the remobilisation of a mid-crustal clinopyroxene mush (534±46 MPa) by hot, mafic magma causing variable resorption of mush-derived crystal cores. Towards the end of the eruption, waning magma supply led to less efficient mush remobolisation and mixing, resulting in homogenous crystal populations. Our results highlight that clinopyroxene Cr contents and sector enrichment can be used to track mafic rejuvenation and magma evolution throughout eruptions, while also reflecting spatial heterogeneities within the plumbing system
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