1,721,194 research outputs found
Crustal Magmatic System Evolution: Anatomy, Architecture, and Physico‐Chemical Processes
No full textDegree of sector zoning in clinopyroxene records dynamic magma recharge and ascent
Full text linkThe 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 textTowards 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
Full text linkTextural 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
Petrological constrains on the coarse-grained, high-Mg basaltic enclaves of Capo Marargiu (Sardinia, Italy)
No full textWe present results from a textural and geochemical study conducted on calc-alkaline volcanic and hypoabyssal rocks from the Oligo-Miocene Capo Marargiu Volcanic District (CMVD; Sardinia, Italy).
Stratigraphic units of CMVD consist of lava domes, a pyroclastic breccia interbedded with lava flows, and dikes. The pyroclastic breccia is in lateral contact with a low crystallinity (∼15 vol.% phenocrysts), massive hypoabyssal body hosting decimetre-sized, coarse-grained enclaves with porphyritic textures (∼50 vol.% phenocrysts).
These crystal-rich enclaves and the host rock exhibit phase assemblages of clinopyroxene + plagioclase + amphibole + olivine + magnetite + low-Ca pyroxene, and plagioclase + clinopyroxene + magnetite + low-Ca pyroxene, respectively. Clinopyroxene phenocrysts (≤5 mm in size) in the crystal-rich enclaves show two compositionally distinct populations: type 1 clinopyroxenes are diopsides (Mg#85-90), whereas type 2 clinopyroxenes are augites (Mg#74-82). Plagioclase phenocrysts (≤1 mm in size) from crystal-rich enclaves and the host rock are normally zoned with An75-96 cores grading to An50-75 rims. The composition of amphibole phenocrysts (≤20 mm in size) is Mg-hastingsite. The mineral texture is poikilitic suggesting early crystallization of amphibole with respect to plagioclase. In fact, the primitive (∼Mg#76), high-T amphiboles include clinopyroxene, whereas the more evolved (∼Mg#65), low-T phenocrysts host plagioclase. Amphiboles are also surrounded by characteristic reaction coronas, consisting of tiny microlites (<5 μm in size) of clinopyroxene, low-Ca pyroxene, plagioclase, magnetite and ilmenite. Olivine occurs as phenocrysts (≤1 mm in size) with Fo79-87 cores surrounded by Fo66-79 rims.
The bulk-rocks of crystal-rich enclaves are high-Mg basalts (i.e., 10 wt.% MgO), whereas the host rock is a more differentiated basaltic andesite (i.e., 5 wt.% MgO). Major oxides and compatible trace element modelling suggest that the basaltic andesitic magma originates by crystal fractionation of olivine + clinopyroxene from a high-Mg basalt [1]. In turn, compatible trace elements in the high-Mg basalt are low (330 ppm Cr, 130 ppm Ni) relative to picritic arc magmas, as the result of crystal fractionation of olivine ± Cr-spinel from a primary magma at depth [1; 2].
Thermobarometric calculations on phenocrysts from the crystal-rich enclaves in equilibrium with the high-Mg basalt yield pressures (600-400 MPa) and temperatures (1200-950 °C) consistent with phase diagrams derived by experiments conducted on primitive arc liquids [3]. Nevertheless, (i) the breakdown of the opacitic amphibole rim [4], (ii) the late appearance of plagioclase, and (iii) the correspondence between ∼Mg#65 natural amphiboles and mineral compositions experimentally-derived at 200 MPa [5], indicate that the crystal-rich enclaves experienced a decompression path started at higher crustal depths. In this view, the high-Mg basalt and the basaltic andesite represent two different regions of a chemically zoned magma chamber formed by crystal fractionation of a primary magma ponding at ∼500 MPa. Subsequently, buoyancy forces associated with density gradients caused upward migration of the basaltic andesite carrying portions of the adjacent high-Mg basalt to shallower crustal levels.
References:
[1] Yamamoto M (1988) Contrib Mineral Petrol 99:352-259
[2] Eggins SM (1993) Contrib Mineral Petrol 114:79-100
[3] Foden JD and Green DH (1992) Contrib Mineral Petrol 109:479-493
[4] Reagan MK et al. (1983) Bull Volcanol 49:415-434
[5] Sisson TW and Grove TL (1993) Contrib Mineral Petrol 113:143-16
Physical-transport properties variations within carbonate-bearing fault zones: insights from the Monte Maggio Fault (Central Italy)
No full textPhysical properties of fault zones vary with time and space
and in particular permeability variations are strictly related to
fault zone processes. Results from previous laboratory studies,
conducted on sedimentary fault rocks outcropping in Central
Italy, show that permeability ranges from 10-16 m2 to 10-20 m2,
(Agosta et al., 2007) in response to the intrinsic petrophysical
properties of the material, state of stress and fault rock
microstructures.
Here we investigate the physical properties of carbonate
samples collected along the Monte Maggio normal Fault
(MMF) that is a regional structure (length ~10 km and
displacement ~500 m) located within the active system of the
Apennines. In particular we have studied an exceptionally
exposed outcrop of the fault within the Calcare Massiccio
formation that has been “exhumed” by new roadworks (Fig.
1A). Large cores (100 mm in diameter and up to 20 cm long)
drilled perpendicular to the fault plane (Fig. 1B,C) have been
used to: 1) characterize the damage zone adjacent to the fault
plane and 2) to obtain smaller cores, 38 mm in diameter both
parallel and perpendicular to the fault plane, for rock
deformation experiments.
The MMF shows two types of damage zone (Fig. 1C,D): 1)
a cemented and indurated cataclasite (CC), that extends up the
20 cm from the fault plane and 2) a porous cataclasite (PC),
that is located adjacent the CC.
We performed laboratory measurements of Vp, Vs, and
permeability at effective confining pressures up to 100 MPa in
order to simulate crustal conditions, at the HP-HT Laboratory
of experimental Volcanology and Geophysics (INGV, Rome1).
From ambient pressure to 100 MPa, P-wave velocity ranges
from 4,9 km/s to 5,9 km/s for PC samples, whereas it is
constant at 5,9 km/s for CC samples. Vs show the same
behaviour resulting in a constant Vp/Vs ratio of 1,5 and 1,6 for
PC and CC respectively. Permeability of CC samples is about
10-19 m2 and it is pressure independent; in contrast, it is higher
and pressure dependent for PC samples starting from 10-17 m2
at ambient pressure to 10-18 m2 at 100 MPa of confining
pressure.
Permeability variations are intimately related to fracture
density as well as P-wave velocity. To test the applicability of
laboratory data to in-situ condition we have compared laboratory and deep borehole data by assuming 25 MPa = 1 kmof lithostatic load. We selected four boreholes drilled in central
Italy that have encountered the Calcare Massiccio (CM)
formation: Pieve Santo Stefano 1, Tavullia 1, Villa Degna 1,
and Daniel 1. Borehole velocities show significant values in the
range of 5,7 – 6,9 km/s and this variation is not depth
dependent. The most frequent in-situ values of Vp ~6.3 km/s is
~5% higher than the average velocity registered for both PC
and CC samples at 100 MPa in the laboratory. A detailed
microstructural analysis conducted by using image analysis on
large cores of the MMF damage zone (Fig. 1D) will help in
clarifying the relationship between crack density and geometry and elastic wave velocities and permeability
A new methodology for paleostress reconstruction using theory, field observations and petrophysical data
No full textThe measurement of crustal stress magnitude is always challenging and generally poorly constrained. This is particularly significant in active fault zones where the knowledge of stress magnitude is crucial for understanding fault mechanics during earthquakes nucleation. In this work we propose a workflow using laboratory and field data as a proxy for quantitative paleostress reconstruction along active fault zone. We studied the exhumed Olevano-Antrodoco Thrust Fault (OATF) in Central Italy consisting of a SW-dipping thrust fault that juxtapose middle Miocene carbonates in the hangingwall above upper Miocene foredeep sandstones, W-SW-dipping, in the footwall. We collected 26 samples of footwall sandstones approaching progressively the OATF, from the undeformed deposits (1 km away to the E) to the tectonically deformed sandstones close (50 m far) to the OATF. Field data highlighted that the footwall sandstones dips towards W-SW, thus moving towards the OATF, shallower strata progressively crop out, hence from the stratigraphical point of view, porosity should increase due to the decreasing in burial depth. On the other hand laboratory measurements revealed the opposite. Using a permeameter we measured porosity, permeability, and P wave velocity both at ambient pressure and at increasing confining pressure up to 100 MPa, simulating an increase in burial depth up to 4 km. Porosity measured at ambient pressure decreases moving towards the OATF as well as permeability, whilst P wave velocity increased. P wave velocities obtained during depressurization from 100 MPa to ambient pressure were always higher than those recorded during pressurization suggesting inelastic compaction. In order to reconstruct the paleostresses we started from the Athy's exponential porosity-depth relationship. We calculate the initial porosity at the time of deposition for undeformed sandstones 1 km away from OATF (11.1%) Using stratigraphic and geometrical relationships we calculated that the maximum burial depth of sandstones close to the OATF was about 1500 m. We then calculated that the porosity of sandstones close to the OATF related only to sedimentary load was about 7.4 %. This value is higher than the present-day porosity that is 3.7%. The difference (Δφ = 3.7%, equal to inferred porosity minus measured porosity) is thought to be caused by the tectonic load and inelastic compaction associated with the activity of the OATF that changed permanently the petrophysical properties inherited from sedimentation and diagenesis as confirmed by laboratory measurements. The stress needed to reduce porosity from the theoretical value of 7.4% to the measured value of 3.7% at 1500 m depth, is 64.8 MPa. This value represents the maximum differential stress (Δσ) that acted close to the fault plane (tectonic load). Since field data indicated a compressional regime; this implies that the horizontal stress is σ1 and the vertical stress is σ3. By using the density-depth relationship, it resulted that, close to the OATF at a depth of 1500 m, σ3=37.7 MPa. Consequently, σ1, calculated as σ1=(σ3+Δσ), is 102.5 MPa. Assuming a coefficient of friction for sandstones of 0.71 and overburden-related inelastic compaction in the proximity of the fault plane, it results that the so calculated stresses are exactly the stress needed to reach critical conditions for slip. Since the OATF has more than 500 m of displacement, critical conditions for slip should have been maintained for long time; this strengthens our methodology that can thus be potentially applied for other tectonically deformed zones
- …
