1,721,002 research outputs found
Metasomatismo da fusi silicatici nel mantello superiore: evidenze nelle Xenoliti ultramafiche di Lanzarote (Isole Canarie)
No full textClinopyroxene Growth and Dissolution Rates: High-Pressure Investigation on a Primitive Alkaline Basalt from the Campi Flegrei Volcanic District (South Italy)
No full textWith the aim to investigate the influence of time, temperature, water content and pressure on clinopyroxene growth and dissolution rate, we performed crystallization and dissolution experiments on a K-basaltic rock from Procida island (Campi Flegrei Volcanic District, south Italy). Crystallization experiments were performed at anhydrous and hydrous (1 ≤ H2O ≤ 4 wt.%) conditions, pressure of 0.8 GPa, temperature between 1030 °C and 1250 °C and dwell time between 0.25 and 9 hours. Crystallization experiments show that time is the factor that most affects the growth rate compared to temperature and water content. Clinopyroxene growth rate, indeed, varies from 10-7 to 10-8 cm/s and it reaches the maximum value in the shortest experiments (0.25 h) while it decreases increasing time (9 h). Comparing our high-pressure growth rates with the low-pressure ones available in literature related to clinopyroxene, it was possible to note that pressure does no affect the growth rate. Indeed, all the considered growth rates show similar values that vary from 10-5 to 10-9 cm/s regardless of pressure but as a function of time. Moreover, partition coefficients based on the crystal-liquid exchange demonstrate that the chemistry of minerals progressively approaches to equilibrium from the shortest to long-lasting experiments, putting forward the latter as representative of the ideal condition of crystallization in a deep magmatic reservoir. Short experiments, instead, could be representative of ascent mechanisms in disequilibrium conditions and quick times. Dissolution experiments, instead, were carried out at high pressure (0.8-2 GPa), superliquidus temperatures and different dwell times by using the seeding technique. Preliminary results show that clinopyroxene dissolution rate varies from 10-2 to 10-7 cm/s, highlighting an influence of temperature and time with respect to pressure
A New Model to Estimate Deep-level Magma Ascent Rates, with Applications to Mt. Etna (Sicily, Italy)
No full textMagma ascent velocities, v (dH/dt; where H is depth and t is time), can be determined from decompression rates (dP/dt), and rates of cooling (dT/dt): Graphic, where ρ is magma density, P is pressure, T is temperature and g is the acceleration due to gravity. This equation for v provides a key to investigating the relationships between the initial ascent velocity of magmas and the depths of magma dehydration. Ascent velocities can be calculated using pressure and temperature (P–T) estimates from mineral–liquid thermobarometry and cooling rates inferred from crystal size distribution (CSD) theory. For recent Mt. Etna lava flows, both dP/dT and dT/dt have been characterized for the portion of the feeding system between the Moho (∼27 km) and 6 km based, respectively, on clinopyroxene thermobarometry and clinopyroxene CSDs. Deep-level (>6 km) magma ascent velocities range from practically zero (where clinopyroxene P–T estimates form a cluster, and so dP/dT ≈ 0), to about 10 m h–1 for flows that yield very steep P–T trajectories. Many lava flows at Mt. Etna yield P–T paths that follow a hydrous (∼3% water) clinopyroxene saturation surface, which closely approximates the water content inferred from melt inclusions. Independent assessments of deep-level water contents have been obtained by means of a new geohygrometer and yield ascent rates of ∼1 m h–1, in agreement with the slowest rates derived for magma effusion or vapor-driven ascent (∼0·001 to >0·2 m s–1, or 3·6–720 m h–1). Changes in P–T slope, as determined by pyroxene thermobarometry, indicate an upward acceleration of magma, which may be due to the onset of deep-level magma dehydration linked to the non-ideal behavior of water and CO2 mixtures that induce a deep-level maximum of water loss at P ≈ 0·4 MPa and T ≈ 1200°C for a CO2 content >1000 ppm
Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)
No full textNew insights on the petrology of submarine volcanics from the Western Pontine Archipelago (Tyrrhenian Sea, Italy)
No full textThe Pontine Islands form a volcanic archipelago in the Tyrrhenian Sea. It consists of two edifices, the islands of
Ponza, Palmarola and Zannone and the islands of Ventotene and Santo Stefano, respectively. The Archipelago developed
during two main volcanic cycles in the Plio-Pleistocene: 1) the Pliocene episode erupted subalkaline,
silica-rich volcanic units, which constitute the dominant products in the western edifice (Ponza and Zannone
Islands); 2) the Pleistocene episode erupted more alkaline products, represented by evolved rocks (trachytes
to peralkaline rhyolites) in the islands of Ponza and Palmarola and by basic to intermediate rocks in the eastern
edifice (Ventotene and Santo Stefano Islands). In this paper we present new geochemical and petrological data
from submarine rock samples collected in two oceanographic cruises and a scuba diving survey. The main result
is the recovery of relatively undifferentiated lithotypes that provide further insights on the magmatic spectrum
existing in the Pontine Archipelago, allowing modelling of the whole suite of rocks by fractional crystallization
processes. New major and trace element data and thermodynamic constrains (by the software PELE) indicate
the existence of three distinct evolutionary trends corresponding to a HK calcalkaline series in the Pliocene,
followed by a transitional and then by a shoshonite series in the Pleistocene. In particular, the transitional series,
so far overlooked in the literature, is required in order to explain the genesis of several peralkaline felsic rocks
recognized in the Archipelago. On the whole, the new geochemical data i) confirm the orogenic signature of
the suites, ii) allow to rule out an anatectic origin for both subalkaline and peralkaline rhyolites and iii) indicate
highly heterogeneous mantle sources, due to crustal components variously recycled in the mantle via subduction
Clinopyroxene growth rates at high pressure. Constraints on magma recharge of the deep reservoir of the Campi Flegrei volcanic district (south Italy)
No full textClinopyroxene growth rates were experimentally determined in a K-basaltic rock from Campi Flegrei Volcanic District (south Italy). The primary objective was to provide constraints on the clinopyroxene crystallization kinetics at high pressure: we carried out a series of experiments at 0.8 GPa, 1030–1250 °C, 1 ≤ H2O ≤ 4 wt.%, with durations of 0.25, 3, 6 and 9 h. Overall, growth rate reaches a maximum value in the shortest experiments (~ 3·10−7 cm·s−1), decreasing to ~ 1·10−8 cm·s−1 in the longest duration runs. Temperature and water content do not seem to significantly affect the growth rate. Moreover, partition coefficients based on the crystal-liquid exchange show that mineral chemistry progressively approaches equilibrium with increasing run duration. Furthermore, to estimate the magma recharge of the deep reservoirs, we combined the determined growth rates with data from thermobarometry and from crystal size distribution analyses of clinopyroxenes in the most primitive scoria clasts of the Campi Flegrei Volcanic District (CFVD). We obtained a minimum residence time of ~ 5 months for the larger clinopyroxene population, and an ascent velocity of ~ 0.5·10−4 m·s−1 for the CFVD K-basaltic magma. The smaller clinopyroxene phenocrysts and microlite populations, however, suggest that the late stage of Procida magma crystallization took place in disequilibrium conditions
A new model for estimating deep-level magma ascent rates from thermobarometry: an example From Mt. Etna and implications for deep-seated magma dehydration
No full textWe propose a method to estimate deep-level magma ascent rates, and potentially, the depths of initial magma dehydration, using pressure
and temperature (P-T) estimates from mineral-liquid thermobarometers, and cooling rates inferred from Crystal Size Distribution
theory. Ascent rates can be ascertained by recognizing that the slope of a given P-T path (dP/dT), rates of cooling (dT/dt), and
magma ascent rates (dP/dt) are interrelated, and ascent velocity, v, is given as: v = 1 ⎛dP⎞ ⎛dT⎞, where Ú is magma density and
Ú g ⎝dT⎠ ⎝dt ⎠
g is the acceleration due to gravity. Preliminary applications of this method are provided for Mt. Etna lava flows, where both dP/dT
and dT/dt have been well characterized based on, respectively, clinopyroxene thermobarometry, and clinopyroxene CSDs (the latter
yields dT/dt = 2×10-6). Deep-level (>20 km) magma ascent rates range from effectively 0 (where clinopyroxene P-T estimates form a
cluster, and so dP/dT ≈ 0), to about 10 m/hr for flows that yield very steep P-T trajectories. Many lava flows at Mt. Etna yield P-T paths
that follow a hydrous (3% water) clinopyroxene saturation surface, as calculated by pMELTS (Ghiorso et alii 2002), which closely approximates
water contents obtained from melt inclusions; these slopes yield ascent rates of ~1 m/hr, and are comparable to the very
slowest rates derived for magma effusion or vapor-driven ascent (~0.001 to > 0.2 m/s, or 3.6 to 720 m/hr). The initiation of such upward
movements, however slow, may be key to understanding eruption triggering mechanisms. At Mt. Etna, certain flows exhibit two
kinds of clinopyroxene crystallization behavior: from a single flow, those clinopyroxenes that form at the greatest depths either follow
a clinopyroxene saturation surface (as calculated from pMELTS) or cluster along such a curve, but clinopyroxenes from these
same flows that yield more shallow depth estimates fall on near-vertical P-T paths. Such changes in slope appear to indicate an acceleration
of upward magma transport, which may be due to the initiation of deep-level magma dehydration
The oxidation state of spinel-peridotites from the Hyblean plateau and the modeled composition of coexisting C-O-H fluids
No full textIntergranular Melt Inclusions within Ultramafic Xenoliths from Baker Rocks and Greene Point Volcanics (Northern Victoria Land, Antarctica)
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